Neville was so desperate not to fall into the trap every ex Liverpool player turned pundit did, and be so blatantly bias towards his favoured club that he would become a meme but went way to far, becoming the complete opposite and arguably the most anti United pundit on TV.
He's so far gone I don't think there's any way back. While other ex United pros have been negative over the years, think Scholes as an example, they have also been able to enjoy things more recently as we have improved, even letting themselves hope.
Neville doesn't. He can rarely bring himself to be positive about United and the rare occasion he does so, there's always a but.
Someone made a post, either in here or elsewhere about how Neville is the only pundit you couldn't guess who he supported, if you didn't know before hand. And that just about sums him up. And its not as if Sky demand it from him, everyone else is able to express their support.
I can't remember how it ended for him at United other than he fell off a cliff and was absolutely horrific in his final games. So it's not as if he can feel wronged by the club or Fergie as Keane was (who is still able to separate his personal feelings for one man with his love for the club often enough).
Thinking about it, it's could be something as simple as what a lot of United fans on here seem to suffer with, after years of disappointment and negativity, it's basically become the default setting.