gee, I didn't know Nautilus had an aoe stun, nor did I know either of them could dash over walls and permaslow.
no Amumu is so popular because he has TWO stuns and an aoe stun that you don't need any aiming to pull off; only thing you need is to be in the middle of the enemy.
truth be told with only so many buffs and ccs in a game you will see a lot of champions having fairly similar kits with some twists to make them unique.
so you play Sejuani because you like playing Sejuani.
same reason you would play Xerath, LeBlanc, Veigar, Fiora and other champions around the 2% popularity mark.
not a single one of them are weak; they're just not popular due to them being hard to farm/get kills/counter/play with or having hard counters in the current meta (garen vs fiora, anyone?).