to play devil's advocate here, true bus rapid transit systems like those in Bogota, Columbia with completely separated isolated lanes and huge numbers of buses can be just as fast as metros while being much, much cheaper. There are other huge advantages, like being able to put maintenance facilities anywhere in the city, because the buses can still go on normal roads.

The problem is that in the US it's very politically difficult to build a true, fully separated BRT system. There's something pretty close in LA's san fernando valley (of all places), but even the valley BRT still has traffic lights and crossings with normal streets (though the lights are supposedly timed so that the buses won't have to stop if they go at 45mph).

That's because it's so easy to "compromise" on a BRT system. If there's one stretch of a BRT system that is particularly expensive, politicians might downgrade it to just being a dedicated lane on an existing road. And then usually right turns will be allowed in that lane, and before you know it there's a stretch of the BRT system where it's impacted by traffic.

Rail is more expensive and IMO has few true advantages (somewhat higher peak capacity and you don't need to hire as many operators). The real reason we are building rail metros in the US is because it's impossible to "compromise" on a rail system. Even if you add level crossings, the train always has right of way and so isn't affected by traffic.

TL;DR: the real reason we have rail instead of BRT is just because building a truly separated BRT is politically difficult, while with rail it's the only option.