Well, for many reasons.
- It may have something to do with the fact that he returned and said he'll never leave again... then immediately left.
- Getting handed the WWE title
- Rock/Cena 2 was terrible
- Rock wasn't the same that he was during his previous run.
In a nutshell, this is why. Some agree with it, some disagree, some feel the fans are full of it, some feel they're right on. Whichever side of the fence one falls on, these are generally the reasons why.
The few times since the post WrestleMania Raw that The Rock's name has been dropped, its been met with boos or a very mixed combo of cheers and boos. The novelty of The Rock coming back in the manner that he's come back has worn off for a lot of people, especially with many fans believing his last run to be lackluster.
When it comes to The Rock, I can see both sides of the issue. The Rock has helped the last three WrestleMania ppvs draw more than 1 million buys, his merchandise sells well and he's someone that's a genuine Hollywood movie star, but got his start as a pro wrestler. The Rock's helped WWE make a lot of money the past several years and while some aren't wild about the idea of The Rock coming back and being handed a plush WrestleMania spot, The Rock is someone that's repeatedly demonstrated the legit star power to warrant being given the royal treatment.
On the other side of the coin, as I alluded to earlier, I think the novelty of The Rock's appearances have worn off for a lot of people. Here we are more than three years after The Rock returned, he declared he was "never going away again", and people realize that it's pretty much bullshit. He's come back during the most lucrative time period for WWE, is given a prime spot with tons of television time for WrestleMania, is essentially handed the championship to defend in a match against John Cena in which the outcome is immediately known the moment The Rock took the title from CM Punk, is given such a prime spot ahead of guys who've been busting their humps for years and disappears for the next 8 or 9 months until WrestleMania season rolls around again the whole thing, as far as the level of his attention & focus goes, to start all over again. As I said earlier, in The Rock's case, it's at least understandable when you consider his level of star power. At the same, some fans are still irked by it regardless of The Rock's star power. When you compare The Rock in 2013 in his matches with Punk and Cena, The Rock's cardio wasn't anywhere close to where it needed to be. I give him credit for gutting it out when he was injured in the middle of his match with Cena but, at the same time, The Rock was also pretty much gassed.
IF The Rock returns to WWE for next year's WrestleMania in the role of a wrestler, it's entirely possible that he'll wind up getting the Batista treatment.