Snoke killed off Kylo's soulmate as soon as he found them, or maybe Kylo just never had one. Hux does, though. Purely by chance he meets them, and before he can even consider his options Kylo cuts them down in a murderous, possessive rage.

bonus:
+++++ Hux didn't really care one way or another about soulmates -- it's a horrible concept in reality and his parents are a prime example as to why -- but it turns out it feels AWFUL to have your soulmate killed, especially in close proximity. Thanks, Ren!