Because 99% of open source software has about 1-3 real devs and after they give up there is no one who can be bothered to take over. Several older projects like X11 and CUPS have no real developers anymore.
Yep. It's trivial to fork, not trivial to maintain. Especially when it's something where security is important - mosh will allow anyone to connect to the server directly if there is a bug there.
There is 600 forks on github.
How many of these 600 forks have unique code i.e. aren't just snapshots of the canonical repository? A lot of people use the fork button as a way of bookmarking or mirroring in case the canonical repo disappears.
That's because Github, in their early days, got very confused about what forking was and made banners saying "Fork me on Github" when what they actually meant to say was "Star me on Github".