As with most things, it’s just history. Roundabouts were invented here in the US, but the inventor made a tiny but critical mistake. Originally drivers inside the roundabout had to yield to drivers entering it. Obviously we know now that this leads inevitably to gridlock during heavy traffic, but back then it wasn’t so obvious. The result is that roundabouts were written off as a bad idea, and signalized intersections (also invented around the same time) took off instead.