I think it has to do with local conditions and the local infrastructure. Let's take 3 examples:

1. The US: large territory, lots of airports, lots of people traveling (due to high living standards), practically no trains (a bit on the East Coast, but overall an insignificant amount), good highways but low speed limits. This situation calls for many flights between many different points and the logical choice is small planes. Regional Jets galore.

2. Europe: better infrastructure offers the possibility to avoid planes in some cases (high speed trains, highways that allow driving fast). So for short hops there are sometimes alternatives and that significantly cuts down on the need for feeder flights on very small planes. That's why for European airlines RJs are fairly rare and mid-range planes (B737/A320 family) are common instead. There are stil a large number of big cities, significant amount of travel (thanks to high living standards) and significant competition in the airline industry (there are more airlines than in the US) - all that calls for increased frequencies rather than fewer flights on larger planes. There are no intra-EU flights on wide bodies that I know of.

3. Asia: fewer airports, concentrated in large, densely-populated cities. There are fewer major airlines than in Europe, but more than in the US. The level of competition is somewhere in the middle. The infrastructure is fairly poor in most places (Japan being an exception) but it's catching up. For now, trains and roads are not a good alternative yet, which means that flying is still the norm for even shorter distances. Lower living standards in most places cause (a) fewer people to fly and (b) diminished flexibility (in terms of timing) is required. Wide bodies still make sense, but in some time (say 10-20 years) I expect intra-Asia flights to move towards a situation that resembles Europe.