A moderator explained to me in a separate thread: Microsoft believe that the lack of apps for WP8.1 is the reason why it never took off. They've produced a couple of software tools which help developers port their Android and iOS apps over to W10M.
However, in order to make this porting viable they've largely abandoned the WP8.1 design language and made the W10M user interface much more like iOS and Android, including common controls like the hamburger menu.
Personally I think they are wrong about why WP8.1 never really got a toehold in the market, and I don't believe this approach will boost the available apps by much.
In particular, now that W10M is an Android look-alike the obvious reaction from a user would be to stop messing around and just use the real thing - Android!