Why were the Diadochi kingdoms so ineffective at fighting the Romans? It seems that Pontus, the Seleucids, and Macedonians lost nearly every single battle against the Romans. It's an atrocious record for a military system that was so successful before.

In comparison, Germans, Gauls, and Carthaginians were able to inflict significant defeats on Roman forces. 

Why were the Romans able to dominate the phalanx-based militaries so easily, compared to their other adversaries? Can it be reduced to the phalanx being an outdated tactcial system, or were their other significant weaknesses that caused the decline of the phalanx effectiveness after Alexander?