I build (mostly) iOS apps as a freelance developer. From my perspective, I've had clients successfully gage product-market fit with just iOS, so as to not double costs building and subsequently altering both. It's cheaper, and as grezql says, it's simpler/cleaner.
This certainly depends on your business strategy and preference though, there are lots of good reasons to build for android, even first. But if you have done the business calculus and think you can get away with just an iOS build, you probably can.