To get developers (and existing apps!), but of course also because
there was something they liked about Amiga. I mean, they could made it
compatible with Linux instead, but they didn't.
I agree that MorphOS and AmigaOS4 represent two different branches,
springing from the same "stem", that is AmigaOS3.x. Some people will
be interested in only one of these branches, for various reasons,
others will have an interest in both.