Relationship between Functor, Applicative Functor, and Monad

Question

When reading up on type classes I have seen that the relationship between Functors, Applicative Functors, and Monads is that of strictly increasing power. Functors are types tha

Answer 1

Changing the definition of Monad at this point, would have broken a lot of existing code (any piece of code that defines a Monad instance) to be worthwhile.

Breaking backwards-compatibility like that is only worthwhile if there is a large practical benefit to the change. In this case the benefit is not that big (and mostly theoretical anyway) and wouldn't justify that amount of breakage.