* It's becoming increasingly difficult to find new contributors who want to work with very old code bases in languages like C or C++. Some open source projects have said they rewrote to Rust just to attract new devs.
* Reliability can be proven through years in use but security is less of a direct correlation. Reliability is a statistical distribution centered around the 'happy path' of expected use and the more times your software is used the more robust it will become or just be proven to be. But security issues are almost by definition the edgiest edge cases and aren't pruned by normal use but by direct attacks and pen testing. It's much harder to say that old software has been attacked in every possible way than that it's been used in every possible way. The consequences of CVEs may also be much higher than edge case reliability bugs, making the justification for proactive security hardening much stronger.