It is all about the flex - just like with dualy trucks.
Double wheels cut the flex by 1/2 if not more as this is not linear function.
US build trailers are build with very loose suspensions, so flex is huge.
That is why European vehicles have their tow capacity cut to 1/4 what they can do with European trailers.
Any sedan can tow 5000lb of European trailer, but those trailers do have totally different build and no problem to have less than 200lb tongue weight.
See the axle placed about 1/2 length of the bed? The driver claims to reach 120 kph on occasions. You will never see it on US build trailer.