SpaceX - Why Not RS-27A?

SpaceX recently announced plans to develop the
"Falcon 9", with a first stage powered by nine
Merlin engines each producing 85,000 lbs thrust
at sea level for a grand total of 765,000 lbs
of thrust.

Merlin is the first hydrocarbon rocket engine
to be so nearly-fully develop in the U.S. since
the Saturn V F-1. If NASA agrees to abandon
Delta II and its RS-27A engine in favor of EELV,
Merlin will become the most powerful hydrocarbon
engine in the U.S..

No matter what happens to SpaceX long term,
Merlin is a commendable accomplishment.

Still, I have to wonder if it might be easier to
develop a rocket powered by four 200,000 lb
thrust RS-27A engines than to create a nine-
engine cluster of smaller engines. Since Boeing
is disposing of Rocketdyne, RS-27A (which is
already a proven cluster machine) should be
available for commercialization beyond the Delta
family.

So why not use smaller numbers of a proven engine?

- Ed Kyle