Building a subway is indeed a very big and expensive, but you need to remember that building the tunnels is the most expensive part - and that kind of risk is usually not logical, taking a young city, undeveloped and unpredictable. It's just better for them first to see results, then invest again.
Anyway, what you really seem to not understand is why a sub is built. It is built to serve cities that are very developed, highly-populated, which have a very high-density which causes a lot of traffic, and the solution - a sub system.
Nir.
