So they made Android mostly free software. The thing is that at some point they ended up having the biggest market share with operating systems connected to the Internet. So the idea was at this point to make Android non free. But it was already free software, so how do you do that? People can also fork. You do it through the control of google play. They stopped developing many applications in Android and continued developing them in Google Play

edit: a relevant 2018 article - https://arstechnica.com/gadgets/2018/07/googles-iron-grip-on-android-controlling-open-source-by-any-means-necessary/