"I wonder how much of HERE not being available on Android is down to Google wanting people to use Google Maps"
"Most of it."
I would say none of it. Google don't seem to care if another map provider wishes to ply their wares in the play store. There are some excellent ones, at least as good as 'HERE'. Try Sygic for instance (paid) or one of the free openstreetmap based ones.
However, it may well be more down to people wanting to use Google Maps, or even Waze.. For many the plentiful (or unlimited) data plans combined with reasonable cell coverage and Maps' ability to cache the route makes it good enough for free. HERE would be competing with that by charging people, and when you charge quite a lot there are many alternatives in the Play store as mentioned.