Someone better qualified will probably answer this but I'll have a crack.

I would suggest it wasn't the formation itself that was so unique about the phalanx - effective as it could be. IIRC the persians fielded spear armed formations too for example (sparabara?). Rather the unique feature was the city-state's capacity to field such a formation in volume/quality - the quality of the troops training, their equpment and cohesion. So while a persian formation might have something like the assets of a phalanx, the Greeks could do it better because of the nature of their society/system. A bit like how English longbowmen were not easily copied by other kingdoms - fielding them required a pretty specific social and political structure or you end up with a dud.