The United States has, on average, an order of magnitude less people per square mile than most european countries. In other words, 10 times less people for the size.
That means we are very big, and very spread out. So for any non-urban area, far fewer people are going to use a given mass transit system, and even for city to city travel, in most cases it would have to travel farther to carry the same number of people.
I’m not saying mass transit doesn’t work ever. It’s *great* in dense areas. But keep in mind that in most cases, mass transit will be substantially more expensive per person in the US than in europe, so comparisons of cost and quality need to take that into account.