My thinking on why there aren’t taxonomies for pages is this.
Posts, on the other hand, are intended to be a revolving type of content – out with the old, in with the new – which means there would naturally be a lot more posts than pages and, thus, there would be more of a need to taxonomize posts, rather than pages.
I think there could be some cases where site owners could benefit from taxonomized pages, but, they’re probably in the vast minority (and I’d wonder if CP was even the right tool for the job if that were the case.)
The best determination on whether to use categories or tags will solely depend on the content; categories are hierarchical, tags are not. So, if you have content that needs subcategories, use categories…if you don’t need subcategories, then use tags or categories. There’s probably very few (I can’t think of any) good reasons to use both and, in fact, the mess that it usually creates is bad for SEO.
I’ve written a guide to understanding categories/tags in ClassicPress, if you’re interested. https://codepotent.com/classicpress-categories-and-tags-demystified/
~ posted by Code Potent