I think it has to do with they want to stall 5G adoption until they figure out how to get American companies at the heart of 5G standards.
I believe that I read a while ago that royalties on standards patents accounted for roughly 40% of the cost of a DVD player, while the companies that actually manufactured them made razor-thin margins. Most of those standards were owned by American or Japanese companies.
If IoT and 5G really lives up to their billing, then the big money from 5G is going to be made by the companies who will collect royalties on the standards for everything which incorporates them, which means nearly everything made for any purpose (look at the latest overpriced crap from Nike to see IoT shoes for example). The American government will see it as their duty to ensure that it is American companies collecting that economic rent on nearly everything made and sold. They have said repeatedly and clearly that they want the US to be the global "leaders" in all things.
Keep in mind that this is the same American government who claim that the Canadian and European steel and aluminium industries are a threat to national security and imposed massive import tariffs on them, and are preparing to do the same with automobiles.
If there were genuinely some serious threat posed by Huawei equipment, then I would imagine that we would be seeing some actual evidence for it by now. There are enough independent security researchers out there that could have found something if there were even a hint of something to find that was at all credible.
If the American government have some clear evidence, then let's see it. Just saying "trust me" isn't going to convince many people outside of the US about "national security threats" whether we're talking about bars of aluminium, cars, or 5G equipment. The American record so far doesn't exactly inspire trust.