It was almost certainly a matter of policy (pre-STS-51L, there was still quite a strong push towards the shuttle taking over all US launches) rather than strict necessity. Some flights (STS-41D, for instance) launched civilian comsats which surely can't have had any terribly exotic requirements.

It was the first of the Lacrosse satellites; the remainder were launched on the Titan IV, though the first launch of that wasn't until a year later.