It might also go back to something as simple as the fact that, when the Slayer was first created, the Shadow Men were only aware of, or only cared about a threat to, their own community, not realizing that the demon/vampire problem was actually global, and more than a single person, regardless of gender or power, would be capable of handling alone. By the time that they did come to realize that, either the original Shadow Men were dead, and the activation spell had been lost, or it was a very limited spell, capable of supporting only one Slayer at a time. That would also explain why the Watchers would be so determined to locate, train, and pretty much dominate the one Slayer at their disposal. They considered each girl, the recipient of their "gift" of power and mission, to be pretty much their personal property, to use, abuse, and expend as they saw fit.
It might be the mysterious "Coven" that is referred to in later seasons who actually locate the Potentials and new Slayers for the Watchers, and wouldn't it be ironic if those ladies actually were or were agents of the Guardians, secretly advising the Slayers via Slayer Dreams and other subtle ways right behind the Watchers' backs? They might have even gone so far as to be responsible for the power that the Gypsies used to ensoul Angel (the "one moment of perfect happiness," I have no explanation for that wouldn't serve as a Spuffy jab, and that's not what my intentions are here. Honest. I swear, Really) and the Cave Lurker who restored Spike's soul, knowing that Buffy would be needing those particular heroes to assist in her fight.
AND the offshoot sections of rogue Slayers might be some sort of attempt to restore the natural balance in the Slayer force, using bad Slayers to counterbalance good Slayers, thus dropping the odds down to closer to the "one girl in all the world" ratio.
