The idea of phalanx was probably dropped as inferior tactics. And then on the northern frontiers they relied more on paying off local chiefs than by organizing military. That's how the foederati became a thing and that's how duxes and rexes emerged. They had their own way of conducting warfare and those whom they fought had very similar tactics. Which could be very flexible so even if a central will had existed to train the soldiers/warriors on the borders to use phalanx as their basic formation/tactics, the adversaries could just retreat, run circles around them, and bit them while they were on the march. I also think most battles were more liek the size of skirmishes, until the Huns arrived.