No particularly good reason. I have no use for the vlc gui, so mpv is fine for me. At a push I'd perhaps say the lighter dependency stack is a plus, but if mpv didn't exist I'd probably install vlc without thinking about it. FWIW, apt wants me to install 36 new packages for vlc even with --no-install-recommends.

edit: Or - I guess - some other scriptable media player, as being extensible is the thing that matters to me. Unless someone magically matches my quirks with their non-scriptable player ;)