First things first, think of the greek cities as seperate countries. Each did things differently.

Sparta was the most militirised city-state, followed by Conrith. Historically, it was very rich, as it had conquered other areas. As such, not as high a proportion of it's CITIZENS were farmers, and as such, it could have a permernant army rather than a hastily-raised hopelite phalanx.

Spartan hopelites trained from birth to fight; part of their training was killing slaves in the street. No other city-state practiced this, and as such they don't have spartan hopelites.