Because users are idiots. "Our map works best in these cities and the experience isn't the same outside of them" is answered by "1 star, sucks, needs bus data

Because users are idiots. "Our map works best in these cities and the experience isn't the same outside of them" is answered by "1 star, sucks, needs bus data in Ohio". Mobile apps nowadays need a lot of good will to drive usage, so if any part of the experience is even slightly substandard your users will rake you over the coals. So, completely reject any user for whom the experience isn't great - it's the rational move. Sadly, supporting your edge cases is harmful to your own health.