My headcanon is that magic is like tides in the Buffyverse, magic (and its manifestations) ebbs and flows, waxing and waning like the moon. For the most part, magic is hard to do and come by, but there are times when the stars are right, prophecies are ready (to me, prophecies in the Buffyverse seem more like rituals that take generations, because it's not so much that they come true but that when all that happens as it said it should then it becomes possible for the events to happen, like a very complicated spell, which is why the cults and demons are so busy in making sure all happens as the uber-ritual demands), and in these times magic becomes more common, easier to use, and that also leads to more demons, prophetic conditions, and the like.
Heck, an apocalypse often has signs which suggests the natural world is breaking down with more demons and occult manifestations, which I'd think would also mean more magic. Is it the apocalypse doing it, or is it the magic making the apocalypse more viable?
Buffy got called as the tide of magic was still rising, and peaking by the time of season 5, which is why so many plans came to fruition since people as the Mayor and Glory finally had the magic they needed (though granted, more was still needed that they saw to), and many secret societies (and Wolfram & Hart) probably had their own secret projects going on adding to the ambient magic as they were tapping the same forces as the Mayor and Glory and the Master. Note that the Master and that demon Balthazar (the one in serious need of a stairmaster) were coming to power at about the same time as the Mayor meaning, to me, that the magic was lifting all the boats, not just the Mayor's, and not just Buffy and Willow. But before that they were all forced to bide their time (ranging from decades to centuries) as the magicks, while still present, were too weak.
Heck, you could say that it was still rising after which helps explain the retconned magic (so that Willow could suddenly phase people into other dimensions or turn them into animals with Amy on a whim, standards that would've seemed absolutely ridiculous in season 1, or even season 3!). Furthermore, it would also explain why things like unique magical urns could be found on ebay by Willow as normally they did people no good so were more curiosities, and thus easily attained by those who realized "the rules have changed and those work again." Even Anya mentioned more than once about magicks not used in over a thousand years were being used now. This would also explain many of the things shown in season 5 Angel as I'd think the world would be different otherwise. For that matter, compare the Wicca group at UC Sunnydale in season 4 and 7. The rising tides of magic could explain the very notable differences between them!
