Because VBA won't do full screen in MacOSX and is incapable of running fullspeed on a G4 1.33ghz whereas Richard Bannister's port of Boycott Advance will do both (though the emulation is a bit buggy).

It's good enough for Hikaru no Go 1 though and that's all that really matters :)

(until I beat it and want to play Hikaru no Go 2, which apparently doesn't want to work)