There might be some license issues, but the dirty secret is filesystem portability isn't terribly important, and for the users for whom it is - ExFAT and friends are usually good enough.
exFAT is widely used but it not being journaled has led to so many thousands (if not more) people losing tons of data, many of which wouldn't have lost so much data had they used a journaled filesystem (or even one with redundant file tables.)
If you need to connect a portable drive to machines of different OS's, there is no safe filesystem that supports read and write on both Windows and MacOS.
Alternatively, cloud storage works until the files are larger than the space you have left on Drive/Dropbox/OneDrive/etc., and local network sharing (on certain networks at least) is more complicated than what the average user is willing to put up with. In practice, many use USB flash drives or external HDDs/SSDs with exFAT. Yeah, people should have more than one backup, but we know in the real world that's not what many do. That requires them spending more time (e.g. configuring local network sharing or setting up an old machine lying around to be a NAS) or money (more on cloud storage.) In practice, having a cross-platform, journaled filesystem would lead to a lot less data loss.
Aside from exFAT, the only alternative with native cross-platform R/W capability is FAT32, but while it has a redundant file allocation table (unlike exFAT), it has a max file size of 4GB, which limits its usefulness for many workflows.
