Why this Fork?
MPlayer wants to maintain old code, even if it is very bad code. It seems mplayer2 was forked because MPlayer developers refused to get rid of all the cruft. The mplayer2 and MPlayer codebases also deviated enough to make a reunification unlikely.
mplayer2 development is slow, and it is hard to get in changes. Details withheld as to not turn this into a rant.
MPlayer rarely merged from mplayer2, and mplayer2 practically stopped merging from MPlayer (not even code cleanups or new features are merged)
mpv intends to continuously merge from mplayer-svn and mplayer2, while speeding up development. There is willingness for significant changes, even if this means breaking compatibility.