If you focus primarily on one aspect, or even several aspects, of anything - you will let other things fall by the wayside. It doesn't take very long, either.
Going by historical records (books, especially, given the time frames) there used to be a lot more Hapkido-type techniques in TKD, which were not present when I started - we had to perform a 2 minute demonstration of releases, controls, joint locks, falls, etc., for black belt, but such information was not actually in the testing requirements at any of the lower ranks, nor was there a specific curriculum for it. It was something we did rarely - perhaps several times a year. Such things have since been added back into the curriculum - but we're adding things to it as we go, as we weren't taught it as juniors; we've had to go out and learn it elsewhere and bring it back.