[Silhouette]

You've almost answered your own question, I think. Providing mass transit on popular routes at peak times is relatively easy. It's more difficult when you need to get someone from A to B that is 100 miles away, and then back again the same day. It's more difficult when you are getting someone from A to B at the start of the evening, but their shift finishes at 4am and then they need to get home again.

To provide a viable transport network, operating full-time with competitive journey times, without making a prohibitive financial loss or being environmentally unfriendly, you need a critical mass of people using each service you run. That generally means you need a high enough population density over a large enough urban area that almost all routes become "main routes" and almost all times become "busy times".