A number of reasons, I believe.
STC has cloned the 8051 pinout, and targets replacing 8051 education training modules.
The STC8051U name reminds users this is a 'original 8051' compatible part, and generates publicity.
Of course, those who designed in a STC8H8k64U part might like to upgrade to a 34k RAM part, with the same pinout, and right now they have no pathway.
So it is a trade off.