Why are there GPS MOCAs?
MOCAs provide minimum obstacle clearance, as you note; MEAs also provide this—and more.

MOCAs are determined essentially the same across the board. The question that I think you should ask is why is the MEA higher than the MOCA in some cases where you might not expect it, namely in the case of some GNSS MEAs.

Firstly, all MEAs take into account multiple factors, including those described in the following. Note the mention of airspace buffers and communications requirements.

From the FAA's Instrument Procedures Handbook, on p. 2-35:


  The MEA is the lowest published altitude between radio
  fixes that assures acceptable navigational signal coverage
  and meets obstacle clearance requirements between those
  fixes. The MEA prescribed for a Federal airway or segment,
  RNAV low or high route, or other direct route applies to the
  entire width of the airway, segment, or route between the
  radio fixes defining the airway, segment, or route. MEAs
  for routes wholly contained within controlled airspace
  normally provide a buffer above the floor of controlled
  airspace consisting of at least 300 feet within transition
  areas and 500 feet within control areas. MEAs are established
  based upon obstacle clearance over terrain and manmade
  objects, adequacy of navigation facility performance, and
  communications requirements. 


Earlier, on p. 2-16, the requirement for communications reception in the case of GNSS MEAs (at least those on RNAV T-routes) is explicitly stated. This makes sense given that these two factors are the primary factors needed in flight guided by GNSS.


  A GNSS MEA for each segment is established to ensure obstacle
  clearance and communications reception.


I'm not sure that this answers your stated questions, but I do think it should ultimately answer what you are wondering.