The clear control flow of imperative programming lends itself well to implementation and analysis of algorithms in a teaching environment. Object-oriented programming is a convenient extension of that, so it is naturally what gets used most often. Functional programming (declarative programming of any kind really), on the other hand, is a completely separate paradigm that requires a whole new set of considerations (performance and otherwise) many of which are much easier to visualize if you understand imperative programming first. After all, it all boils down to an imperative language in the end.
