Why it is important that software is still maintained
Every non-trivial software has bugs and security issues (vulnerabilities). Therefore it is important, that the software is still maintained. A program for which no one feels responsible can become a security nightmare.
This doesn't mean, there have to be regular releases. There are programs which are “feature-complete” and there are no noticeable bugs anymore (although such software is very rare), but someone should feel responsible in case, that a security issue will be found one day.