Serbia barely had a second party, as far as I could see (from Croatia). Yeah, there was some opposition, but very few had any sincere trust in it, incomparably fewer than the number of Americans who actually like and support Democrats. The Student List tapped into this vacuum.

And European democracies are generally a bit more loose with having newcomer parties being at least somewhat viable, for better and for worse (AfD, UKIP...). The US electoral system just erases 3rd parties wholesale, it's particularly abysmal.

Btw, the Student List is not terribly left-leaning. They're trying to be "apolitical", in order to attract both the left and the right.