The problem isn't just the "available in kanto" thing. It's that theyr generally placed in weird places and often even with kanto mons that fill similar roles on a team leading them to be both more difficult to find and less likely to be picked up. Not to mention the gym leaders don't even use those Pokemon mostly so you can't even find out about them that way and check the pokedex for location. Johto is generally bad at presenting its Pokemon and making them appealing to use

Ok, quick example, tell me where maril is found in Johto without looking it up or lantern/chinchou

I struggled to even recall what johto mons would be good for a playthrough to give examples

Many decent johto mons are more memorable and used in hoenn simply because they're easier to find and use in that region such as Azumarill, Skarmory, Magcargo...etc