… the USA is not strong enough to risk a war with both the Houthis and Iran at the same time. It can barely continue to militarily confront Iran; adding a campaign against the Houthis would not just tax the US military greatly, it would probably fail. The US has neither the leadership nor the societal unity now to see such a war through. That it had to abandon an ally at this point is simply a reality of that weakness, as Saudi Arabia has discovered (again).
If US weakness was exposed nakedly by this move by Trump, his behavior starting in a few hours will make it even clearer. The visit of President Xi of China, which is about to start later today, will make it obvious that right now the US needs Chinese help more than China needs that of the US. Indeed, the weakness of the US vis-a-vis Iran and the Houthis is connected to this meeting, as China is, as we speak, helping Iran rebuild its military and has helped the Iranians attack US military personnel. Still, Xi will be feted, Trump will boast about how much they like each other (Xi will not) and in the end Trump will be asked to make major concessions.