Associated data can be completely arbitrary (and thus possibly not equatable), I expect the designers would rather people use pattern matching instead of the more error-prone boolean constructs, and likely they don't want to add "unnecessary" capabilities so if the developer don't need equatable enums there's no reason to make them and developers can ask for it when they need it.

Rust makes the same choice, so does haskell, though in both enum/data equality is just a derive/deriving away.