Adding someone into a solo project introduces social overhead. That's not necessarily a bad thing and can even be a very good thing (I much prefer making music with other people and seeing how our influences mix as opposed to making music solo). But there is a cost/risk there which I could certainly see some solo developers just not being interested in taking on.
On the other hand, since this is OSS, if an organization does need N>1 badly enough, then they can commit their own resources to it, forking if necessary.
On the other hand, since this is OSS, if an organization does need N>1 badly enough, then they can commit their own resources to it, forking if necessary.