I really like Forza Horizon. I have Forza 4 too but the track and circuit racing doesn't do it for me. Horizon reminds me of my favorite Need For Speed, NFS Carbon. Forza is developed by Turn 10 who is an in house MS studio, so will always be Xbox exclusive. Unless you mean exclusive to XB1 I'm which case that's so people buy the new consoles.
I'm ticked that Titanfall and Ryse are XB1 exclusive but they're by Dice, an EA subsidiary so with luck they should come out on the PS4 in time. I'll end up getting both consoles as usual though.