I think its down to a rather strict pay range set in place for youngsters at United. The general consensus amongst the United fans I know is that Pogba, like the others yet to break into the first team squad properly and consistently have to stick to a certain wage limit, with the promise that if they progress they will earn huge $$$$ and be playing for one of the best teams in the world. So to a degree, YES, it seems that its come down to money. I'd also chuck in the friendship between Pavel and Mino Raiola...as well as the fees which the move will bring to Raiola's pockets.