Someone’s already replied you as to the meat of why it doesn’t make sense on Alliance, but I thought it was pretty common knowledge by now that the Horde helps Vorrik and his people a great deal more than the Alliance through the Vol’dun questing zone. I played through the foothold quest on my Alliance character and it was basically, ‘oh, strangers from shore, can you help me get my rock back rq? And btw there’s some catastrophic thing about to happen. Kthnxbai.’
Hordeside, after you clear the zone and the dungeon, you can see a priest of Sethraliss in Dazar’alor. While this is likely just flavor, it also shows that the Devoted are establishing a relationship with the Zandalari.
Also, as said before, Sethraliss is a loa. A troll entity of worship. While this doesn’t preclude a faction joining (see: void elves, Elune and the Church of Light being different things), it makes it significantly less likely as nearly the whole Alliance has some sort of beef with some sort of troll.