This is no surprise, wordpress is extremely popular these days for all users, a huge angular2 client framework might be an overkill for many of its users. Drupal on the other hand is doing the more complicated websites, and its "ordinary" user base is shrinking, so angular2 might attract enterprise users.
Laravel is arguably the most popular PHP framework today, choosing vuejs as its client framework gave vuejs a strong boost for sure.
As for Vue vs Riot, it's subjective, but I'm personally partial to Riot due to its simplicity (albeit with a smaller feature list) and focus on components without being opinionated at all (not even regarding the toolchain).
I like to think of Vue as React with sensible conventions. That said, both are terrific and solve UI problems very elegantly.