mplayer is simpler for me given my low-requirements:

* less/simpler compile-time dependencies (no libass/shaderc/lua/…)
* no dedicated project just for building it
* no need for configuration at all (so better configuration doesn't matter to me)
* I had experience with remote URL openning fine in mplayer but not mpv (direct links to the video file!)

So I kept using it as it works well for my needs in the end. I don't need/want an API controlled or scriptable video player. And as I compile it myself, the simpler the better.
And mplayer is still actively developped :)

To be fair, I tried to use ffplay instead at some point, but it is just not a media player at all…