I know very little of the technical details. But it begs one question and answers yours.
MPEG4 was introduced 1998 and is more efficient than MPEG2. If for no other reason to use it , it saves bandwidth. So more information can be compressed into a smaller amount of data and return the same picture quality as MPEG2.
Hence it was a natural considering it has been around for 10 years to use it in any new standard. I actually expect that MPH will have a pretty nice picture on such a small screen.
The question is begs is why does
ATSC use MPEG2 ?
ATSC was standardized in 1996, and two years later MPEG4 was standardized. There was plenty of time even in the early predicted transition dates for
DTV, the
ATSC standard could have been upgraded to MPEG4. In 1998 few stations were on the air, and virtually no receivers were available. Now we are stuck with an
OTA standard that is 11 years old. Why I constantly say
OTA DTV is not state of the art, just a 13 year old standard that took forever to implement.