"The Normans invaded England around a thousand years ago and took most of Wales at the same time, and their descendants kept successively chipping away at the increasingly small remaining bits until they basically ceased to exist, hence for constitutional purposes Wales is part of England."
That's either because we're (sheep) lovers, not fighters... or as Irish comedian Andrew Maxwell once said:
The Welsh slowed down the aggressors with confusion:
"sorry. I can't fight youse. I'm busy fighting me mate, Barrie"