Efficiency is the highest output for the lowest "cost", cost being opportunity (GCDs) & resources (cooldowns & mana).

Cure is currently 400 potency, for a single GCD.
Regen is 1050 potency over the course of its duration and the same opportunity cost (it also allows for oGCD skill weaving for EVEN GREATER opportunity efficiency, but we'll ignore that variable).

So regen, for one GCD, provides ~2.5x the potency of Cure for ~1.4x the mana.

Cure spells are for getting a person's health up RIGHT NOW. They're inefficient, but very powerful on a moment to moment basis.
Regen skills are "weak" when looking at a single moment in time, but massively efficient if they are allowed the time to do their jobs.

This is why Medica 2 and Regen are our workhorses, not Cure 1 & 2.