Why did the romans changed from Hoplitewarfare

[Tambarskjelve]

Well these are all valid points, but none are really confirmed.
The greek historian Polybius was the first to describe the manipular legion, about mid 2. century BC.
It was then already an established military doctrine, albeit more dependent on spears than the later legions.

It is speculated that the Romans adopted a deep figthing formation because of terrain, partly influenced by Samnites, and lessons learned establishing the northern borders. That this formation proved succesful in the Macedonian wars ensured it's continued use.
It is also speculated that they adopted the 4 maniple formation to differentiate the tiered roman society.

Roman society was competitive to the extreme, so a need to single out who was excelling in single combat was there, in addition to having 3 differently equipped hoplites/legionaries, based on wealth and social standing.
The citizens mustered on Campus Martius every fifth year, where the tribunes would take their pick in turns to the legions. This was also where one could make claims to climb the social ladder, by appealing to a Censor, who would evaluate the Citizens property, gear and accomplishments. Thus the manipular legion was the very core of Republic society.