Aren't almost all questions on meta primarily opinion based?
The literal answer to the title question is in all likelihood because nobody bothered to adapt the close-dialogue for the meta site.
That said, also for a meta questions there is a scale of "opinion based"; the cut-off should certainly be very different on meta than on main, but the notion to some degree still makes sense, which justifies what I proposed in the first sentence.
For the question in the body, literally, no, not almost all question on meta are a matter of opinion. Many a support question is fact-based. (Note: this answer is fact based :-).)