The American voting system is not Proportional Vote and instead is massivelly Mathematically rigged with (for Congress) huge single representative electoral circles which in some cases have borders designed exactly to make it near impossible to defeat incumbents (aka Gerrymandering), with (for the Senate) even larger electoral circles (literally, each state) with 2 representatives, and something somewhat similar for Presidential elections (though worse since ultimatelly it all adds up to a single representative electoral circle with 300 million voters for a position with lots of power, unlike most European countries - with some notable exceptions like France - which either don't have a President or have one which is mainly symbolic and has little power).

Further, the very nature of the system will, beyond the Mathematical rigging, push the people who would otherwise go for a 3rd party to instead go for the "useful vote" (i.e. chose an electable candidate instead of the one they want) - it's not by chance that the heaviest argument of the Biden campaign was "vote Biden to defeat Trump".

Since new parties take various electoral cycles to grow, it's pretty impossible for them to do so because it's Mathematically near impossible for them to even establish a foothold that shows its earlier supporters they do have a chance to one day influence what laws are made in the US and how the country is ruled, so new parties invariable lose steam after the first or second election they go through.

You can see something similar to this in the UK, were for example the Green Party gets 1 million votes out of 40 million (2.5% of votes) but only 1 member of parliament out of 300 (0.33%), and remember this is with lots of people chosing electable candidates from other parties, so the Green Party natural vote would likely be larger in a different system

This stands in marked contrast with, for example, The Netherlands, were vote is Proportional and there are 15 parties in their Parliament (Tweede Kamer).