Why? Wouldn't hindlimb mass (in the form of muscle) still be useful for
those animals that used a leaping launch?
An excellent point. Modern prosimians that leap from trees rely on their
hindlimbs to provide sufficient impulse to launch themselves into the air.
This favors legs that are strong as well as long. This holds for aerial
leaps that are launches from branches, and from trunks. In the latter case,
the primate begins the launch with its back to the target landing site, so
it has to rotate its body during the launch phase