What's your source? That seems extremely specious to me, for two reasons.

First, the US does not technically have a two-party system. If your standard is how many parties have seats in the national legislature or parliament, the US has three parties represented since Justin Amash is now affiliated with the Libertarian Party, as well as a few independents.

Secondly, many countries only have two parties that really matter in terms of having a realistic chance of forming a government--the UK and Canada are two examples that spring to mind. The third largest parties in those countries are the SNP and Bloc Quebecois, respectively--parties who want some specific part of the country to have greater autonomy if not independence. If your complaint is that the US doesn't have a Texas Independence Party winning seats in Congress, weird flex but okay.

Virtually every polity that uses a first-past-the-post voting system will end up with a two-party system. That's Duverger's Law. The primary counterexamples only prove the point, because they more or less replace one of the two national parties on a local level. (I used to live in Seattle, and while Seattle politics are officially non-partisan, Seattle has actually developed a de facto two-party system between mainstream Democrats and socialists.)

I personally favor multiparty systems with proportional representation, but that's no guarantee of anything. Israel (which somehow still ends up with a de facto two party system anyway) and Belgium have both notoriously failed to form majority governments for long periods of time.

The typical third party in Canada is NDP, which has had support between 15-25% of the vote within the last decade depending on the election. Which is quite different and quite more than the Libertarian party in the USA which has a very negligible influence. They also have been in power within a few provinces quite frequently. I don't think Canada was your best pick to make your point.

Canada has only ever been ruled by the liberal or conservative parties

NDP won fewer seats than Bloc Québécois in the last election. And it’s worth pointing out, I think, that the most prominent US politician who would be aligned with the NDP ideologically, Bernie Sanders, is actually an independent rather than a Democrat.

> NDP won fewer seats than Bloc Québécois

That's mostly because Bloc Québécois is concentrated in a specific province, and often specific parts of that province, while the NDP's support is spread across all the provinces.

It's the same in the UK, where the Lib Dems got triple the vote of the SNP, but less than a quarter of the seats.

Yes, as I pointed out, this is an artifact of how Duverger’s Law guarantees that first-past-the-post voting systems lead to two-party systems. BQ is one of the two parties in Quebec just as SNP is one of the two parties in Scotland.

Independents won almost 20% of the popular vote in the 1992 U.S. general election. Also hold more seats in Congress. That’s a more apt comparison, not the Libertarian Party.