We
discussed it on github and decided to first test it on mobileread and XDA-developers before bringing it to Google Play.
Koreader was originally for Kindle only, then it was ported to Kobo devices in mid 2013. The 7 devices with their ever changing firmwares are now well supported. However, the port to android reaches a multitude of devices and different firmwares a magnitude larger. So koreader needs feedback on how it runs and on the problems that arise.
We are getting feedback and bugs are ironed out. More feedback is much appreciated!
If you have an issue with koreader, post it
here
For an impression of the speed of development, you could look at the latest nightly releases
here.