Game mechanics and engine.
Height is crucial for a GK in fifa.
That's why all the best GK's in the game are 6'4 or taller.
It's why I hate Julio Cesar, I don't know why anyone rates him as a decent keeper for Brazilian teams.
I'd much rather have Cech with Luiz or Courtois with IF Miranda or Sirigu with Silva or Neuer with Dante or Santana.
OVR (ratings) also means a lot to a GK but if you have say an 86 rated keeper who is 6'1 and a 84 rated keeper who is 6'6, I'd pick the 84 every time what he lacks in rating he makes up for with height and then some.
The best example of this is trying to get a small keeper to win the ball at a corner.
I've never seen Julio Cesar win the ball comfortably and most of the time he will be beaten in a 50/50. If it's inside the 6 yard box even then he has a decent chance of claiming it otherwise he's useless for defending corners.
Handanovic is the best GK in Serie A in fifa 14. Sirigu is also really good if you have 2 italian CB's.