It is better because it has not changed a lot in the last 20 years. It may be true that there were some incompatible revisions somewhen in the 90s, but for all practical purposes that is history with no practical importance today.
Also, simply because of its age, you can be sure that RTF does not infringe any patents, because *all* relevant patents have expired. So you can legally create a RTF-reader/writer, even when you use the Microsoft-RTF-specification (their specification is copyrighted of course, but you don't copy the specification, you just use it.)
With OOXML all that is not so clear.