I think it's probably a combination of the not needing the teacher anymore, and of the distrust he created in "Lies", and of a couple of seasons of him already establishing that he wants Buffy to stand on her own.
Then you've got the 500 slayers in training they're all working with, which is probably making them very busy.
Buffy's growing control-freak-ism-ness may not have a lot of room for Giles at this stage in her life. Plus, it's noted that the distance she felt increasingly over the years is just intensifying now because of the nature of her work.
What I see in their distance from each other, though, is that Joss has created a need for reunion/re-meeting of minds between them, and hopefully that will happen.
I sometimes wonder if maybe Giles found out about Buffy pulling off that heist, and they had words as a result.