There are a number of reasons developers develop for iOS first, or avoid Android completely. Perhaps the top two reasons are OS fragmentation and device fragmentation, both of which drive up the development costs (more use cases to build for, more use cases to test for).
This post from GigaOM makes the case that Google has solved these problems, at least in part. The key is Google’s Play Services.
Play Services, introduced in 2012, is effectively a background download of core services required to run apps on Android. Putting the OS install numbers to one side for a moment, this is the stat that matters to developers – over 93 percent of all Android users are running the latest version of Google Play Services.
More importantly, Google has been slowly moving core Android features, APIs and app elements out of the OS and into Google Play Services — meaning developers can ensure their apps run smoothly (with all the new features they plan to implement) across all devices carrying the latest infrastructure.