Why do you think not as many people have used characters created by Morrison after he was done, like the Ultramarine Corps or Manhattan Guardian? It's sad.
It’s a habit, though while some ideas being passed over seem to be genuinely a matter of spite and hatred for what he did (almost everything done with the X-Men since New X-Men) or laziness (Solaris never showing up), a lot of the time I do get it. The Batmen of many nations, even though he made them great, are difficult to justify bringing in all that often if you’re still doing stories all-but-exclusively set in America. The Ultramarines and the Wanderers and Justice Incarnate are the type of pre-fab concepts where if the creator themselves don’t follow up on it and develop it further, most likely no one else will either. Most of the minor characters he works on aren’t ones the Big Two are going to be chomping at the bit to take a big chance on. And of course a lot of the time it’s a matter of writers being confused - whether it’s actually all that confusing or if they just don’t get it is another matter altogether - or thinking his stuff is too silly or weird to continue using. And then there’s characters like Vyndktvx or Doctor Hurt who are so tied into specific stories he told that it’s hard to imagine them outside that context. Between Al Ewing, Tim Seeley, Jason Aaron, Steve Orlando, Tom King and even Pete Tomasi, among others, we seem to be in a minor renaissance of writers who realize Morrison was onto something and are dragging all that gold back into the light, usually in a pretty satisfying fashion.