Not why they joined the Horde FYI the reason they joined the Horde is quite literally explained in their Questing zone… They only joined them initially to survive and use them to get back to Kael’Thas on Outland.
Till it turns out their King went literally insane and they were kinda like “Well these Horde guys helped us survive and the Alliance just wanted to spy on us…”
On a more meta-level Blizz admitted they gave Belves to the Horde to give the Horde an ‘Alliance’ like race to encourage more people to play the Faction. The whole story and plot around why they joined the Horde is honestly… Bad.
Like really bad, but they at least did something with it.