Question

how did the fact that the roman army consisted of poor volunteers contribute to the collapse of the roman republic

Answer 1

The Roman people were based off of their military, they trained relentlessly so they could not only defeat other nations to gain land but to defend themselves against attacking armies. If they couldn't have trained men then they died.

Answer 2

So, if your army consists of untrained and unskilled volunteers - the only chance you have is winning in numbers. However, sometimes even that isn't enough. If you don't have strong, well built, and skilled warriors to defend your nation - an attack could leave you vulnerable to a collapse.