Its simpler than that.
I have used both db in my time as a developer and I have been mad at both for one reason or another.
It all boils down to Mysql is not as stable and has several thread safety and memory leak issues when its used in a multi-threaded app like FreeSWITCH. In fact that preference is irrelevant because technically the default database in FS is sqlite because the design goal of the project is for the software to run anywhere in a minimal capacity and be able to grow and scale with the user's needs. So anyone can use any database they want. However, when we get a bug report that shows Mysql crashing, that is not something we can do anything about. Since we do have better luck with postgres, we allowed the inclusion of native api but that was a 3rd party contribution.
So bottom line is we don't care what other people do, we just do what we like doing and share it.