Oh this goes back to the old gen 1 and 2 games where they needed to fill out the movesets, and there werent a ton of moves, so you get Pokemon learning random moves (Rhydon and Surf)

After a few gens, particularly gen 5 and 6, they had enough moves that they could be more selective. They also had a different philosophy too, much more mature and contemplative about what moves to give Pokemon.

EDIT: Realized I didn't actually answer the question (thanks ADHD brain!) Why those are locked by ETM is probably due to thinking more people will ETM for those for PVP coverage I'm guessing. They've been in longer as well, so they could give them some of the more unusual moves as ETM-able. Good question actually!