They just weren't able to pull it off for whatever reason. I actually ran BeOS as my daily driver for quite a while (way) back in the day. BeFS was genuinely amazing, and not something I've seen replicated elsewhere yet. There hasn't really been anything interesting done in filesystems used by users on devices in a really long time.
BeOS was mostly a single-user system, and filesystem was optimized for that use case. Having to consider that an opened file for reading may be modified by a remote system running with a different user complicates things, so while I do agree that BeFS was quite ahead of his time, it focused on a somewhat specific use case; At the time, you wouldn't expect that you could eg. correctly backup your email client data without shutting down the application.