Because it's the one you can reasonably suggest to all your utterly nontechnical friends and relatives to get them to stop downloading random sketchy exe's from the internet to install codecs.
mpv, and mplayer before it, has always been better for me and I find VLC to be pretty shoddy. But when it comes to people like my mom, I don't hesitate to sing praise for VLC. In that context, VLC is the greatest thing since sliced bread and I don't muddy the waters by even mentioning mpv.