Why the mighty Western Roman Empire could not withstand the onslaught of tribes living in tribal systems?

The Western Roman Empire was not so powerful even then. It has weakened significantly due to frequent political intrigues, as well as due to constant pressure from the barbarian tribes. The barbarians were very numerous. Rome simply did not have so many troops in order to protect all borders from invasion. Ultimately, the Roman troops bled out of blood in constant wars and the barbarians were able to easily seize the lands of the empire. So in 476 the western empire ceased to exist.