Explanation: The Hellenic states which the Roman Republic fought, and overcame, were centered around the Macedonian-style phalanx, in which troops with 15-20 foot pikes formed a near-impenetrable front line of presented spears. And it was nearly-impenetrable - as long as they didn't have to do anything like 'turn quickly' or 'move over rough terrain'.
The Roman maniple system of mobile units of ~120 men was not a surefire path to victory - there were certainly some battles when the Romans were defeated by the phalanx. But on the whole, battles are unpredictable, and generally the side which can adapt to changing conditions - or accurately predict their opponent's deployment - is better-placed to win than the side which is not.