The thing about technology is that it doesn't sit still. Like my teenage son, technology gets bigger and more complicated every day. So we are continuously updating our software to keep up with the changes to iOS, to various security and privacy issues, and for new Apple devices.
The thing about Android is, compared to Apple's 20 or so device models out there, there are tens of thousand device models running Android! Which makes it a whole lot more complicated to support.
But Google is making things easier for developers, so we will get into the game and release an Android client soon.
First we have to lock down our core value proposition and perfect our product within the tame confines of Apple.
Here's a quick article that neatly explains the problem in depth.