The main selling point of mpv, IMHO, is video quality and color correctness. You can get pretty nice results (on par with MadVR on Windows) with --vo=opengl-hq and it's many suboptions.
Minimalism is also something people are interested in. mpv has 1/5 the code of mplayer and 1/6 of VLC but still has relevant features.
They appeal to different kind of users. For example, even if I am in the mpv team, I will recommend VLC to my 'average Joe' friend as I know he would be put off by the CLI usage and config file editing.