I've been musing recently on the anthologies I buy. There's a sort that's noticeably missing from my bookshelf: best of the year anthologies. I think a lot of the reason is I like themes with a bit more substance, such as a sub-genre, or a certain group of authors or some other connecting thread. Best-of-the-years tend to be a little on the random side.
It can also be an issue that the year may have been a poor one for fiction (from my perspective). Some years, I find I don't like a lot of the stories considered the best, because they're not themes I enjoy.
Even for best-of-the-years with tightly defined themes, the best of the best won't have all been published in one year. I'd rather have a book with the best the editors could find from any year.
Of course, someone is buying those anthologies, because they continue to be put out each year. It's not as though it's a problem for me that they are. But when it comes to anthologies I'm exciting about, and I'm saving my pennies to buy, it's always the un-yeared themed ones that do it.