The received wisdom was that for many years it was easier to make money from Apple customers. Despite the fact that there were fewer iPhones compared to Androids the theory was that iPhone customers tended to have more money and were willing to spend to get things. The Android ecosystem was also much larger and you had to test on more phones to get confidence in the performance of your app across the customer base. So from a developer point of view, you're solving a simpler problem for more money.

In practice actually iPhone 'fragmentation' is surprisingly bad in terms of how UIs would render unexpectedly on different phones - still probably easier than Android but levels the playing field a bit more, Android market share grew and went more upmarket, cross platform frameworks improved and the whole practice has become less common.