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Pax Marsupiala!

With regard to dinosaurs, I freely admit that any mammal, as long as it 
had developed egg predation "technologies", was capable of tippintg the 
dinosaur balance from: equal number dying as hatching, to more dying than 
One can argue the reproductive efficiency of placental versus marsupial 
delivery, though.  (Speculation) A marsupial that does alot of open-field 
running should perhaps rather be bipedal.  I would argue that the "Joey" 
might tip out of the pouch in a cheetah vs. Thompson's gazelle-style 
chase.  Perhaps there are advantages for 4 legs in this niche.  These two 
considerations together would argue for placentals.  By the way, isn't 
most predation in the open field niche of today on the young, the sick, 
and the old.  One does not usually, in National Geographic specials, at 
least, hear of mothers being run down.  This evidence suggests (if true) 
that carrying baby is not much of a liability!
A pouched baby would be more susceptible to infestation and infection 
from parasites.
But the Virginia opposum is a wonderful example of marsupial ability.  
What _is_ it about this animal that makes it so dominant in its niche? A