Based on what? A personal oath from a dead leader to Thrall, who's no longer in charge.
They've been either ignored or used as cannon fodder by the Horde the entirety of WoW, and their beliefs about nature have been repeatedly violated by the Forsaken and goblins. Their leader was imprisoned and threatened by his supposed allies.
There's no real reason for them to be loyal, because the Horde definitely isn't loyal to them.