Even without patent concerns, exFAT takes more effort to implement (lots of features you might not need) and ends up requiring more ROM space that may or may not be available… even if you want to write files exceeding 4GB to some external media.
e.g. some widget without network connection that optionally logs an audit trail to an attached USB media, could still end up with only a couple hundred kilobytes of soldered-on ROM to store the whole firmware, while wanting to write more than 4GB of audit logs.
