Same goes for Jellicent and all the places it can be found while surfing
It's just how Game Freak wanted the game to be i guess or they just want to make the Chargestone Cave to have stronger pokemon
Hope this helped =)
But seriously, there isn't really a reason, it is just a game design choice
Well, do you expect ALL pokemon to evolve at the same lv. Maybe cause the roggenrola in chargestone cave always lived there instead of going around the region. Same in iccurus city. You can find lv 18 seismitoad there.