I think they don't want to do it because of development costs.
Each platform requires slightly different software and they are already planing on 3 different platforms (PC, PS4, XBOX1), some bean counter probably decided that it would not be worth the money to have teams writing and supporting the PS3 and XBOX360 as well.
It's a bit of a gamble since there are so many older generation systems still out there (about 160 M of them), but I guess they figure that the people that invested in the latest consoles might be more likely to by the game.