The same reason that the PC still has Fifa 07 with a Fifa 10 sticker on the box.
Officially EA Sports say it's because they reckon not enough people have powerful enough PC's to run the new Fifa engine. Unnoficially it's because they can't be arsed. If you want a decent football game nowadays you have to buy a console I'm afraid.
Also, the new Fifa Online which is in closed Beta is intended for people with average PC's or Laptops, I read some feedback from the US Beta and apparently it is just like the PC version that EA already have out on the shelves, utter **** but luckily I've got a PS3 and get my Fifa fix from that.