Collaboration is scary
Collaboration usually requires communication and compromises. The pay off in the long run should come in the form of saved time, higher quality and a healthier ecosystem. But how does this all work out in practical terms? I remember a university course labeled "code is law". Talking about the hard facts that code defines. The topic there was mostly about stuff like how the default settings of apache define the reality of what gets logged and not whatever the governments privacy laws define. At the same time it can make collaboration harder, just because one group got their release out first (potentially rushed?) does it mean that their BC concerns outweigh your design concerns? Even if neither or both parties have released code to think about, you might have some other standards are tastes already developed that collide, how to decide the direction, especially if you have a sense you might end up being the junior partner? Now even if both sides are perfect gentle(wo)men will the end result not be some mushy half of everything compromise with no real world value?
There are lots of reasons I just listed to be scared of collaboration and there are probably a bazillion more. I guess the trickiest part is finding a sense of trust. Trust in each of the parties involved, despite each coming from a different "camp", that they still want to search for the greatest sensible common denominator for the joint user base. In that sense likely the biggest obstacles are not even technical but social .. or maybe the success of collaboration is even coincidence of having the right kind of people meet at the right table at the right point in time feeling adventurous, bold and forward thinking.