Because new epub versions have the Python 3 problem. Every reading system out there supports epub 2, and epub 3, the latest revision, still has poor support years later because it's not a big enough improvement to bother with and too much software is written for epub 2. Plus epub has had a rocky past few years as the standard has been bumped around various committees.
Why? Because of switching to HTML5 (if that indeed happens) or any future revision at all won't see wide adoption? If the latter then what's the reason according to you? Amazon's dominance?