Pogba was chasing games, felt he was good enough to get more playing time elsewhere, at a club he apparently has supported since childhood, at least the Platini and Zidane connection, and at Man United he was not getting the chances he felt he deserved. How would you feel if you were consistently amazing in the youth teams, then the reserve teams, but then watched other players progress from youth to senior level, get more playing time, and worst of all Scholes, at 38 years old, return to the fold in a position you were born for...
The rise in wages is noticeable but too simplistic to focus on for the nuts and bolts of his move. It takes bravery, determination and a huge amount of confidence in your own ability to move countries, sign for a team which has recently emerged as having one of the finest midfields in europe, and challenge for the shirt in that midfield, when a player is 19 years old. The money helped, of course, but its a pittance compared to how much he will earn in the future.
I explained a little more of the transfer a long time ago here>
http://lavecchiasignoranet.tumblr.com/post/18931664165/juventus-sign-man-uniteds-paul-pogba