How did Sparta collapse?

Answer 1

Sparta had a limited number of the armoured infantry (hoplites) which was progressively eroded in battles over 100 years faster than they could breed replacements. They began to use their helots (serfs) to replace the losses, but they were not the same quality heavy infantry, so after the losses of a defeated by Thebes at Leuctra in 371 BCE Sparta could no longer raise a credible army. Although Thebes and Athens were later over-run by Macedonia, Sparta just didn't have the numbers to make a comeback.

Answer 2

It was defeated comprehensively by Thebes at the battle of Leuctra in 371 BCE and as its military population was now so low, it could not make a comeback and concentrated on controlling its own territory.