People who develop new filesystems generally have a problem with existing filesystems and have some goal they want to accomplish.
There is also the issue that somebody else's "existing" file system becomes your "new" filesystem when support comes to your OS. For instance Linux has support for many obscure filesystems such as Amiga and the old mac filesystems because somebody might want to mount an old disk. I don't think anybody really wants to run a volume like that because they want to use it to do their ordinary work on Linux.
XFS, ZFS, JFS to name a few are foreign filesystems that claim to be good enough that you might want to use them on a Linux system not for compatibility but because of performance.
Why are there so many filesystems? Because everybody starts from scratch.
Then everybody writes code first and later a description of the filesystem.
We need a way to learn from previous mistakes and then find a way to fix those problem.