Why is the Nissan Skyline illegal in the USA?

It's not. Infiniti G35/37 = Nissan Skyline (V35/36)

The real reason is this-

Nissan never submitted cars for testing to show that it met US safety and emissions standards.

A second issue is that no one was willing to buy enough cars to submit for testing in order to get a license to import them for road use.

Japan also has emission laws and passenger safety laws, and while not interchangeable the Skyline with a minimum of modifications should be able to meet these.

But Nissan elected not to try and the expense for a private individual exceeds the cost for a different make and brand of performance car that can already be purchased in the US which is equal or better then the Skyline.

Department of transportation side crash protection and bumper rules. Environmental Protection Agency (Emissions standards) for cars that year model. American laws for flammability standards for interior materials. Head light standards. There are few really good Skylines available too.

Skyline does not meet US emissions standards, but that does not mean you cannot import or own one, only that you cannot get it licensed in the US. So you could let it sit in your garage, but you could not get license plates for it or legally drive it on any public roads. I guess if you really wanted to, you could take it to a race track and drive it there, but you'd have to move it on a trailer to get it there.

definite and no..there are procedures around it yet you may would desire to purchase a clean one in japan, your taxed for uploading, delivery cost,cost for enhancing to fed rules,cost and extra cost,, it may be extra cost effective to have a automobile geared up for overall performance than procuring a skyline..

Its too ugly for the US market