What is the problem then?
Mainly straight-line skating, not the greatest physicality, a lackluster shot, and 5-on-5 defensive play.
At this point Koivunen's path to the NHL is basically a spot in the middle-6 as a secondary offensive contributor or nothing at all as I don't really know if Koivunen's game can evolve enough to adapt to a more checking role at the highest level.
And that's not a bad prospect at all to have really, you just have to be realistic. Same for a lot of prospects in the AHL too (a guy like Joshua Roy on the Canadiens comes to mind as being in that same situation given his very clear lack of skating ability despite skill and smarts on his side).
The reality of it all is that only a handful make it despite all hopes. All you can really do is pay attention to the prospects you like and hope everything goes well for them.
I say that, but it doesn't mean Koivunen can't develop further and make many like me look foolish for "pigeon-holing" his potential in the NHL. He absolutely could.