Flexibility matters.
“That flexibility becomes decisive because of what I’d argue is a real Roman equipment advantage, which isn’t really a technological advantage, per se. The only new technology the Romans are using is mail armor, which, to be fair, is very good and I think quite impactful. But if Roman and Hellenistic iron-working or production methods differ, we can’t see it. Instead, it isn’t a technological advantage, but a mismatch that gives the Romans the edge. Roman heavy infantry are heavily armored for the attritional, close-combat the legion is built around, by contrast, Macedonian phalangites, while still heavy infantry, were less well protected, because they were supposed to have those long sarisae between them and the enemy and in any case were a pinning force, not a killing force.”
https://acoup.blog/2024/04/26/collections-phalanxs-twilight-legions-triumph-part-ivc-perseus/