It's a scale issue. There are plenty of small metro areas (600K-1.2MM pop) with affordable housing, plenty of jobs, and absolutely zero useful mass transit. Traffic is bad for about one hour in the morning and two hours in the evening, at most, and usually just in one direction with plenty of side streets.
You double that and the infrastructure collapses. The tax base doesn't support large investments in transit to get a system built from nothing, and the roads and bridges aren't equipped to handle 2x the volume.