Because her story so far makes it seem like her family is a big mystery that will be revealed later. If it turns out that her family are a bunch of no-names, it would be really anticlimactic.
And her name is REY....Gli-funka!
George and Kathleen have said that Star Wars is the story of the skywalkers. So everything is connected to that small family, they're the most important people in the whole galaxy and mostly everyone else is a nobody.
"Her name is Rey...Kenob...son! You know, the Kenobsons from Ord Mantell? I think they're accountants...
I don't particularly care who Rey's parents are. Rey is awesome. I want to know if Finn is Force Sensitive. That's the most pressing issue for me. Abram's comments I read a few months back about how the Force, to him, was about how anybody, if the need and desire was strong enough, could tap into it speaks just as much about Finn as it does Rey. I think we're going to see more Force Sensitives in this new trilogy period, with Rey being the primary protagonist and focal point. I think both characters were tapping into the Force throughout the movie, with Rey actually being aware that that's what she was doing, and Finn having no clue.
If Rey is a Skwalker, great. If she's a Kenobi...Great? If she's a random. Great. Just keep building on her already likable and endearing personality, and I'll be happy. She's very charming, as is Finn and Poe. Who the parents are just isn't as big of a deal to me after seeing what she can do with the Force. Don't waste entire acts on going backwards, when I want to see the story go forward. A quick mention of her parents (or confirmation of Luke being her father) during a single scene, is fine. But I don't want all of Act 1 or 2 being Rey having Force visions of her past.
Also, I mentioned this before, but why does there have to be a twist to her lineage at all? Star Wars has gone to the surprise parentage well already. I think the "twist," if any, will be how absolutely ordinary or expected it's all going to be.
The reason why Empire works as a twist the way it does is because nobody was expecting Vader to be Luke's father. We were outright told in A New Hope that Vader betrayed and murdered Anakin Skywalker (well, we weren't told his name at the time). That event was something that happened a long time ago, and wasn't relevant to Luke's journey beyond him wanting to be like his father. So the reveal that, "Nope, Obi-Wan is a lying liar who lies, I'm your dad!" was a gut punch.
Expecting Rey to be a Skywalker negates the twist. We "know" it's coming. Expecting Rey to be a Kenobi negates the twist. We "know" it's coming. Expecting Rey to be a "nobody" negates the twist. We "know" it's coming. We've basically hedged our bets to the point that a "surprise" reveal isn't all that surprising. Her parents could be anybody, and because of that, it really loses all interest for me.
I almost think her "Me too...big secret," line was a glib tongue in cheek comment to BB8. Rey doesn't seem to remember who she is in every scene afterwards. Why would she be keeping her parents secret from herself? She just knows, deep down inside (and possibly Force influenced), that she's waiting for her family. She dreams about the island that Luke is on. His Lightsaber calls out to her. Her family isn't who she's expecting, and if she does remember them, it will be because it's time for her to unlock whatever memories have been suppressed of her time pre-Jakku. I don't think it's going to be a huge revelation in the same sense of "I am your Father." Just a layer of her past being shown to her.
People expecting some never thought of "twist" are really going to be disappointed.
EDIT: I think the problem with so many of the other theories is that they would require a lot more complexity to convey in a single movie. Ultimately, there are a few questions about Rey's history that needs to be answered, and since it's Star Wars, the answer will more than likely be incredibly simple:
1) Who are Rey's Parents?
2) Why was she left on Jakku?
3) Who left her on Jakku? (if not her parents.)
The answer to those questions need to be simple. No overly complex plots revolving around Clones or Reincarnation, or even secret love children of characters who died 30+ years ago.
Luke being Rey's father could be answered simply:
1) Luke and Mara Jade are Rey's parents (they met post Return of the Jedi. She died due to number 2 below)
2) She was sent to Jakku after Kylo Ren killed her mother and the other Jedi Luke was training (Luke may have been under the impression that Rey was killed along with her mother). She was left with Unkar Plott because he's unassuming, and probably wouldn't draw any attention to the girl or himself.
3) Possibly 18 year old Kylo Ren, in a moment of "weakness" (as he'd see it). Perhaps he couldn't do what his grandfather did, and kill a child, in particular his cousin.
All 3 of those answers are pretty straight forward and simple, and don't require any kind of retconning of TFA or the previous trilogies.
Luke not being Rey's father:
1) Her parents have some import to Luke, Leia, or Han. That would require some explanation as to who they are, and why their relationship to Luke, Leia, or Han was important. The reason why they must be important to either Luke, Leia, or Han (or maybe even the Republic) is important because of number 2 below.
2) Her parents left her on Jakku because? This would require a lot more explanation. Was she displaying signs of Force Sensitivity? If she was, would she not have been sent to Luke's New Jedi Order instead of left on Jakku? If her parents aren't important, then there wouldn't be much need to send Rey away. She'd just be their Force Sensitive daughter on some backwater planet, with nobody really giving a shit about her. She has to be connected to the Jedi Academy in some way. Probably a student.
3) If her parents left her on Jakku, then this question was already answered above, but if her parents aren't important, there really wouldn't be much need to leave her on Jakku.
I just think that her being a random, and her being a Kenobi would require a lot more screen time and set up than what we've seen so far, and what Episode 8 will cover. It'd need its own anthology movie to fill in the gaps (which, I guess is something Lucasfilm could totally do, but the movies should be able to stand on their own as well, and not require you to watch 10 other movies to get; an aside, my niece hadn't seen the OT or PT, but she loved TFA. It wasn't required that she watch those to get the new one).
Her being Luke's daughter wouldn't require nearly as much set up, as I outlined above. There are enough flags in TFA that her being his daughter wouldn't bog down Episode 8 with exposition. She may have questions about her mother, which Luke could answer. Maybe she doesn't like what she hears, or maybe what she hears alters her trajectory as a Jedi, leading to conflict. Maybe her relationship with her father isn't as rosy as anyone was expecting (Luke included). There could be some tension there. Their first meeting at the cliff seemed bittersweet. Either way, Luke has some explaining to do. Rey is important no matter what. Why she is so important is the real question, and that question won't be answered with some crazy over the top theory. It's probably going to be laughably simple, which is why many people are going to be disappointed.