I think Cena winning was pretty obvious. Orton was just recently a huge flop as the champion and it's be weird for him to get the belt since you'd have no direction to go. Reigns and Cesaro are way too early. Del Rio is lol, so is Kane. Sheamus is essentially a mid-carder and fits nowhere. Bray didn't feel like a contendor either after the Cena fued. That leaves your safe option in Cena. This just shows the lack of starpower in the WWE roster. They have Cena, Orton, and Bryan. The other main eventers are all part timers like Triple H, Lesnar and Batista.
They need to make an effort to establish the Shield, Cesaro and Wyatt within the next year because if they mess up there, they're going to be even more stuck in 1-2 years time. So far, they're doing well with them all except Cesaro who seems to be going downhill since Wrestlemania.
The bolded is precisely because they fall back on Cena (and to a much lesser extent, Orton) and focus on him all the time.
For instance, during the NXT/Nexus invasion and angle, Wade Barrett had absolutely no problem fitting in at the top of the card, cutting a 15-20 minute promo to start off RAW, having the spotlight on him, etc. And with the deck stacked for him at... Survivor Series I think it was, with Wade Barrett vs Orton for the title, and Cena as the guest referee (and a member of Nexus, against his will), I remember the build up for the match not being about Wade Barrett and Nexus knocking on the door of greatness, of capturing the title, of conquering the WWE. No, it was "What will John Cena do?" It wasn't about Wade Barrett. It wasn't about Orton. It wasn't about the title. It was about John Cena. The fucking referee.
And Mr. Hustle, Loyalty, and Respect screwed Wade Barrett out of a title win, shoving him into an RKO.
Then they had Cena bury Nexus and Wade, literally. You built up all this momentum for someone, then instead of giving them that one push to stardom, you ignore them, and fall back on Cena. And Barrett's been stuck in the midcard ever since, neglected and forgotten until BNB.
Similarly, Cody Rhodes has always been entertaining in the ring and on the mic, popular, and safe to work with. After Legacy, we got his successful Dashing character, and they went with the logical step of a lengthy IC title run, which really elevated the midcard and the status of the title. But after he lost the title, instead of doing the logical thing and doing a lengthy singles feud with someone in the upper card, or pushing him into the title picture, they paired him up with Damien Sandow and made them a tag team. The push never came. He's 29, so there's still time for him to become a huge star with 5 or 10 years of upper-card credibility (for reigns of his own and rubs to other wrestlers), but he could have been a generational star like Orton, Cena, The Rock, HBK, etc.
There's no reason why Barrett should have fallen off the face of the earth for 3 years after Nexus, or why Cody should have gone from his IC title reign to the tag division instead of a title feud, or non-title feud with someone in the upper card. Let's hope they don't fuck up Bray Wyatt or the Shield like they have with so many others, but after that Cena feud with Wyatt, and with reports from dirt sheets over the last year of HHH constantly having to prevent Vine from dissolving and depushing the Shield and its members... I'm still worried.