The Mezzanine

Meta reason: the hammer was introduced to be used in the finale. When Spike tries to lift it in... "Blood Ties", right? ... it's too early in the season and the hammer isn't ready. Kind of like how you can't open certain doors in a video game until you've killed all the lower level bosses.

Slightly less meta reason: Um... with all those gods running around, maybe Thor decided to cut Buffy a break and make sure the hammer remained where it was? Or something?

But especially in light of the Scythe, your explanation works great. Like you say, Buffy is stronger than Spike, but he can't lift it at all and Buffy lifts it without any effort.