A few reasons:
First it's rolled out as basically a way to test new ways to interact with the product. It's not meant to be the main app at the moment, it's meant for a small subset of users. Second, it's being developed by Mike Matas
-- longtime Apple developer (and former Apple employee) -- and using techniques and interactions that only work fluidly on iOS. The animations and UX stuff they are doing can only be done reliably on iOS. Third, the (unstated) goal appears to be to appeal to a subset of Facebook users that really value design and that experience -- by and large those users are iOS users. It's another part of the reason most apps do go iOS first: it's a better platform to develop for and the users are better users. They use apps more, they value the design and process more, and they equate to "better" users. If you're looking to experiment via your Creative Labs department with with a new paradigm for browsing your product -- developing for a platform with less fragmentation and an engaged userbase makes the most sense. The experimentation allows you to A-B test new features and a new experience without alienating your entire customer base. If only a small % end up using it every single day -- but they're using it because it brings them joy and value when the other app did not (I fall into that category, I use Paper far more cause I really like it -- and I avoided Facebook on mobile before) -- that's a net win. You have engaged and delighted a group you weren't before.
And lastly ... with an experimental release, it makes sense to go small and then expand:
1) The launch is US only
2) The launch is on iPhone only
3) The launch is on iOS 7 only
They went small on purpose to best optimize the experience.
(More from the developers themselves