It said in all the mags that angle was told he could not wrestle as often due to the medication he was on due to his neck injury he didn't like this and left.
I do watch a bit of TNA and it is good but they do have a lot of people from the WWE who have left due to not been good enough to be pushed for titles ie cage they pushed him and he wasn't good enough.
The WWE is a bad a thing for the wrestling world i don't doubt that but it is much better than TNA