> Why do people want to be in that region? Is it the default or something?

It's one of the oldest and largest regions. It hosts the most services, both low-level platform stuff and higher level managed services (which run on the low-level platform stuff), so services tend to be more performant.

Geographic location is also good.

Also, due to scale their pricing ends up being cheaper.

Let's say that it's the region people use by default, unless they have a compelling reason to have a presence in any other particular region.