I think Diaw really craves the structure that disciplined teams like the Suns and Spurs provide. It goes beyond having good teammates or being able to play his game--in Atlanta, he had no idea what he was 'supposed to do', so he was indecisive on the court and never got anything going.
The other thing is, this is Diaw's third year in the league. A lot of players struggle in their first couple of years, then turn it on in the third year (Joe Johnson and Barbosa were two).