The reason BYD is killing it is because they can offer their cars at a price point unavailable to the US. The reason for that price point is because China is producing some of the cheapest batteries in the world.
BYD cannot build their cars in the US because the core part they need to make them cheap is the batteries. CATL makes the batteries that BYD uses and they aren't going to setup shop in the US. A lot of what makes CATLs batteries cheap is because China has a raw materials trade pipeline that's now superior than what's available in the US.
All of this goes back to tariffs.
By putting insane tariffs on all imports the US has effectively isolated itself from the rest of the world. Manufacturing will defacto be more expensive in the US because a significant portion of any incoming raw resources will get an automatic 25% tax.
The US does have it's own raw resources, but they aren't fully developed. Prior to 2024, we were heavily reliant on imports for a lot of our manufacturing. Shaking up the entire market for stupid reasons has destroyed manufacturing in the US. It'll take decades to repair and rebuild.
The steep tariffs against china that Trump did in his first term against solar, steel, and batteries were maintained by Biden. In term 2 Trump ramped those up to 11.