Google has used the excuse that the marketshare is insufficient to devote apps to Windows Phone, and that wouldn't be entirely untrue, except that Google has history of open animosity towards Windows Phone. Microsoft built a fantastic Youtube app that Google shut down because Microsoft didn't receive permission from Google to use some particular APIs in the app. Then there was a few days where the Google Maps website was actually being blocked on google's end by any web browser that had Windows Phone in it's user string.
Microsoft chose not to support WP7 series hardware with WP8. The difference between WP7-WP8 was much greater than WP8-WP10 will likely be. We don't know any firm information about WP10, but I expect that current hardware will be upgradable to WP10.