It's not just OWB. There are a lot of OS4 programs - both backported from MorphOS and ported from other platforms - that had some initial work done but have been stagnant ever since. I wish more OS4 developers understood that maintaining software is just as important as the initial release.
If we're talking MUI-OWB I think kas1e (who did the Os4 port) got pissed off and stopped development on it after the initial release. Something to do with Hyperion iirc, but memory is sketchy, if I can find the thread I'll link to it.