Because we live in a globalized world, things that are popular spread across borders fast. So we want to make it as easy as possible. Distributing an app across the U.S., Australia and Canada makes a lot of sense, because of the minimal language and cultural barriers. An alternative way to approach growth is the so-called horizontal distribution strategy. Instead of climbing up in the ranks, some developers choose to focus on being everywhere.
By creating different versions of their app, they can rank in multiple categories. For example, in addition to its core app, Udemy -- a platform for online learning -- created several different apps for each of their courses. So when you look for programming apps, you will find its programming course. When you look for business apps, you will find one of its business courses, etc.
The downside to this approach is that you lose a little bit of ranking for each of the apps, but on the other hand, you make up for it with your presence across the App Store's ecosystem.