Jeff Findley writes:

Since the SSME is a fuel rich staged combustion engine, you have to
things to start: 1. The combustion chamber for the turbopump (whose
combustion exhaust goes into the engine). 2. The main combustion
chamber. Starting all that up was a complex process, so NASA put as
much of the equipment and consumables to do so on the ground (since
that's the only place the SSME was ever started).

https://en.wikipedia.org/wiki/RS-25#Constellation

From above:

It would be expensive, time-consuming, and weight-intensive to
convert the ground-started RS-25D to an air-started version
for the Ares I second stage.

Jeff

Hence the revival for Constellation (as long as it lasted) of the
restartable J-2 LH2/LOX engine from Apollo. It then re-incarnated as the
new and improved J2-X.

Remember those days. Wasn't the J2-X supposed to be part of the 2nd
stage for the Aries-I 'stick' vehicle? The only version of this that
flew was with a dummy mass simulator upper stage. As the PAO called it
during the launch, "exploring new concepts..."

Dave