War of attrition and a potential lack of future Iranian support. Normally these insurgencies would be able to win the war of attrition but due to their threat to shipping in the area they get a lot more attention and militaries have the will to fight them vs in a place like Afghanistan. Also, they may be losing the war to whoever else is in Yemen (maybe the actual government?) so they need to focus on that and not being a nuisance to global powers.