As expanded on in Who does the Slayer lineage run through?, Buffy screwed up the Slayer lineage by coming back to life.
Somehow, this created an opening that the First Evil planned to (ab)use to put an end to all the Slayers. What I don't understand is why it had to wait until now.
As far as I can recall, its plan was to kill off all the Potentials, then kill the Slayer, preventing anyone from being chosen as the next Slayer. Wouldn't this plan have worked just as effectively before Buffy came back?
If the answer has something to do with events in Angel (including explaining why it waited so long), please hide it under a spoiler - I have yet to watch that series.
To those saying that the First simply lost patience, that is not the case. 7x11, Showtime, includes this dialogue (which took a while remember what episode it was in, hence not being included originally):
Giles: If the first has been around for all time, then why hasn't it attempted something like this before, why now?
Beljoxa's Eye: The opportunity has only recently presented itself.
Beljoxa's Eye: The mystical forces surrounding the chosen line have become irrevocably altered. Become unstable. Vulnerable.
Anya: Something the First did?
Beljoxa's Eye: The First Evil did not cause the disruption, only seized upon it, to extinguish the lives of the Chosen forever.
Giles: Then what has caused the disruption, what- what is responsible for letting this happen?
Beljoxa's Eye: The Slayer.
And when they return to our dimension...
Anya: I just... I don't understand how Buffy's death mucked up the whole Slayer mojo, y'know, it's not like she hasn't died before.
Giles: It's not because she died. The Beljoxa's Eye was quite clear about that in its enigmatic way. It's because she lives. Again. Buffy's not responsible for that.
Anya: Oh.. no. Willow and me. And Xander. And Tara. We're the ones who brought Buffy back, we're the reason the First is here, the reason those girls were murdered. The world would have been better off if Buffy had just stayed dead.
So yeah, I misremembered which death caused the glitch, but it's definitely there, not just a fan theory. The problem is, as far as I can tell, the First's plan should have worked even without the glitch. Why would it not have worked until now?