[caseysoftware]

A big contributor is population density.

I lived in Northern Virgina, just outside DC for ten years. You can look at the surrounding cities' skylines and based solely on the size of office buildings, number of apartments, etc, you can get an idea of where the Metro stops are. The Metro opened 40+ years ago and the buildings clustered around them. Now opening additional lines - like the comparatively new Silver line - is easier because the infrastructure is there, they just have to extend it to the right places.

In Houston, outside the downtown core, things are distributed all over the place. Since there are less "obviously right" neighborhood choices to expand into, you need parking for people from nearby but not close aears. Oh, and since there isn't already the infrastructure, every inch of tunnels have to be dug, track has to be laid, staff trained, etc.

Extending an existing system is relatively cheap compared to starting one from the ground up.

Is that more concrete? (no pun intended.. this time)