One of the keys to making public transit work is population density. It is widely used and popular in densely populated places, and is little uses in places with low population densities.
This is largely a function of technology and economics. Transit systems generally have costs that are only moderately sensitive to ridership, so higher density spreads cost per trip over more people and makes the price of transit lower.
Two good blog posts on the subject are here and here.
The first looks at U.S. transit use by metropolitan area with interactive charts.
The second make international comparison and also introduces the concept of weighted density, which considers the disproportionate impact of densely populated areas within a metropolitan area, and employment density, which considers the impact of employment centers like downtowns and office parks in areas where residential populations are more dispersed.