Charlie Gardner dives in and does some direct comparisons between these two cities that lie across the Rio Grande from each other. It's an interesting piece in that it's primarily an analysis - he doesn't delve deeply into the ten reasons why this is. Some key points:
The economic and demographic convergence at the border might suggest a gradual transition in urban form, but instead there is abrupt break at the Rio Grande river, as can easily be seen above (the map has been rotated clockwise). Matamoros is a city that makes almost exclusive use of attached or nearly attached dwellings on small lots, while Brownsville has a quintessentially American pattern of detached houses sitting on much larger lots.
New homes in Matamoros are often a tiny, in some cases little more than 500 square feet, but appear to be mere placeholders for expansion. Usually set back about 15-20 feet from the lot line, these homes are swiftly expanded forward into the setback and up a story or two using simple construction techniques, with the result that after no more than a decade or so, the street's appearance is completely transformed, and no longer appears mass-produced. (Single-use zoning seems to be unknown or unenforced, as numerous small commercial establishments can be seen cropping up, mid block, along these streets.)
Interesting start to some deeper comparisons, and has some nice photos. The future is likely to see much more of a blending of Latin American traditions and North American, so I hope we'll see more analysis and discussion of items like this. We have a lot to learn from each other, and I'm continually impressed by what the South Americans are doing. For one, they have taught North Americans what a real bus system should look like.