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.