Bret Devereaux

Personally, I think this has to do with the fact that they were mailed infantry. We have a decent amount of evidence for Gallic mounted elites wearing mail, but unsurprisingly such an expensive armor seems to have not been used by the infantry.

Whereas I am quite confident that by the 160s BC, mail was the majority armor-type of the Roman heavy infantry.

What was the harder formation to emulate? The Roman cohort structure of heavy javelin/big shield/stabby stab - or - sarissa phalanx?

I'm with @DrMichaelJTayl1 - it's the legion. The necessary equipment is more expensive and you have to train and prepare for a greater degree of local decision making Sure a file-leader leads his file, but how many units in a phalanx you have individually maneuvering?

Alexander's pezhetairoi had 6 taxeis which might maneuver independently, plus the hypaspists. But assuming we buy the standard cascade attack of the legion (and I think we should), every maniple of a legion can maneuver separately.

So you have that triplex acies w/ potentially thirty separately maneuvering blocks of men per legion, doing substantially more complex maneuvers than the sarissa-phalanx was typically capable of. The number of guys you need who can 'drive' a unit around the field is much higher.

But are those individual pieces doing that much? Attack. Withdraw. Fall back. Each officer is not maneuvering based on the entirety of the battle, but rather their immediate circumstances. Each syntagma probably weighed similar concerns.

I mean, both Cynoscephalae and Pydna are fatal to this argument though - in both cases, the Romans are effectively able to maneuver maniples independently and win as a result, and the Antigonids can't, and lose.