The problem with tags is that we (software devs) know how to use them, but many other people don't. Source: Research done by a startup I worked with.
As opposed to putting "tags" in the app title (motivation for all calendar apps to be named something with Calendar) or X number of times in the description text (motivation for unnatural copy).
Disclaimer: Based on what little I know about search priority in Google Play.
On Amazon's non-app store, you can also buy search ads. Apple could implement something similar and take a larger cut of the paid install market. There are search ads on Google Play (see: http://adwords.blogspot.com/2015/07/launching-search-ads-on-...).