Because he was in the original trilogy, and that's enough for Star Wars purists to think something is awesome: It's on the original trilogy? Then it's great. It's on the prequel trilogy?Then it sucks and must be retconned out of existence.
Just because.
Boba Fett was absolute ass of a character in the original trilogy. He got killed like a bitch, and fans were so obsessed with him that he "miraculously" survived in the expanded universe, which is bullcrap.
In fact, if you ask most if not all of Star Wars' purists about what makes Boba so great in the original trilogy, you only get vague, dumb answers like "He had a jet pack", "he looked cool", "he was mysterious", "he had a cool ship", "his toys were great" and stuff like that. You know, basically nothing related to the character development itself.
Then in the prequel trilogy it is stated that he is a clone, which adds nothing or takes anything away from the original character, but suddenly "IT SUCKS". Why? Because this guy of which we knew absolutely nothing about and died like a bitch, is revealed as a clone. So what? Being a clone or not doesn't change the fact that the guy died like a bitch and was a boring character to begin with.