Auxilia Primary Weapon

[M. Demetrius]

It's also a matter of function. The pilum volley was a very formidable weapon for breaking up an infantry charge, followed by an infantry counterattack. A batch of hundreds, or thousands of pila sailing down on any infantry formation would be very harmful to those on the pointy side of the volley. It would probably be much more difficult to throw a volley of any kind of projectile weapon from horseback, as the closeness of all the horses would be a problem of its own, and there were almost always many times fewer cavalry than infantry.

From some reports, auxilia (often being non-Roman mercenaries, so to speak) were not allowed to use pila as a matter of course, as for the most part, the legions didn't want a rebelling auxiliary unit to throw pila at them. But that may not be factual, just a report.