Couple of reasons:
1: Price with the options I wanted. By the time you add the Recaros, MFT (I like it, OK?), navigation, and a few others, you're looking at a $30k+ FoST.
2: Better fuel economy. I like taking road trips, and if you drive smart, you can get nearly as good economy from a FiST as from a "normal" Fiesta.
3: Size. I live in a neighborhood where street parking is de rigueur. The FoST is bigger on the outside, but not much bigger on the inside. Besides, I rarely have many passengers, and the shorter OAL of the FiST allows me to find more parking spots than if I'd have bought a larger car.
4: Agility. I've been doing a lot of canyon runs with forumites since I got my FiST after moving to Southern California. We'll usually have a few FoSTs in the mix as well, and while they can walk away from us on a straight, they just can't quite hang in the corners. The corners are the most fun for me!