Lack of demand is the conventional explanation. But I think it is a bit of lazy and misleading one. I think there's plenty of demand for cheap EVs. But there's a problem with US manufacturing not being able to deliver those. Supply chains aren't there. Manufacturing capability and capacity isn't there. Etc.
And of course the EV market is still actually growing in the US. It's just that companies like Tesla, Kia/Hyundai, and other foreign companies with factories in the US are picking up the slack left by the likes of GM, Ford, Stellantis, etc.
Protectionism in the form of tariffs and incentives is making things worse. It's temporarily succeeding at keeping competition out of the door but it's failing at making local industry more competitive. Especially in the international market where US companies enjoy neither the benefits of import tariffs nor incentives. They have to compete on merit with the likes of BYD there. And that's obviously going to cause some issues.
Dropping incentives and tariffs would obviously be short term disruptive but I don't think it changes the outcome long term. Which is that GM either catches up or falls over (wouldn't be the first time). Either way, them delaying investments in EVs is not a sign of them adapting. Same for Ford, which has the same problem and is doing the same. Same for Stellantis. They are favoring short term profits over a long term plan. That's because protectionism is temporarily excusing them from having to compete.
That's not something they can dodge long term. Somebody will step up if it is not them.