MPC-BE has several active devs with years of regular work being committed and looks a lot nicer which is reason enough for people to prefer it. Over the past year the activity of users has shifted from mpc-hc to mpc-be it seems, at least on the doom9 forums.

What I would love to see is someone make a nice Windows GUI player based on mpv but depending on your hardware mpv can be a real PITA to get working smoothly without experimenting with various backends so you don't have dropped frames or really fucked up output (my igpu notebook when using my desktops config just produced a green and LSD-trip style static image).