Why aren’t there any large cities in this area?

Literally opened up google maps on a completely random part of that region. Its because 90% of it looks like this.
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Someone could write a lot more, but I think it being dry and cold is the major reason. 

I’ve lived across most of southern Montana at some point or another, including Miles City in the east. There’s nothing there. Some badlands, maybe, which are neat, but nothing to build a city for. Mountains are cool, but most towns there are old mining towns that were close enough to travel routes to survive. Bozeman is growing because it’s a pretty college town and too many people think the Yellowstone show is a realistic depiction of Montana.

No need for a large city if population is low

Not a lot of people so not a lot of a reason to.

Take out Denver and SLC and the area without MSA above 1 million gets a whole lot bigger.

What’s funny is that if you go north of the border Canada has several large cities along the same area. The reason why though is best left explained by people smarter than I, but part of it is that is the least worst option, since everything north is basically uninhabitable.

Cities don’t just happen. They grow up around industry, and usually that means having a port.

The reason most of the biggest cities are on the coasts or along navigable rivers is because things need to get to consumers, and shipping by water is still one of the cheapest ways to do that. So when a suitable place to create a port is found a city quickly forms up around it.

What makes a place suitable for a port is 1) near some industry that requires shipping (mining, agriculture, manufacture) and 2) has, ya know, water. A lot of it.

An example of this is when gold was found in northern and central California in the 1840s. People needed mining gear and the railroads didn’t go that far west yet, so everything had to come in by ship. This led to the development of San Fransisco as a port city. That it was gold also made it the financial capital of the west coast, and thus a big city. But it was the port that made it possible.

None of the area you circled has anyplace to put a port. With rare exceptions, especially in the USA, no port means no cities, especially where there is no large industry to speak of. There is some mining and fracking, but that’s about it. Nebraska has expansive agriculture, but the only suitable place to put a port is right where they did, in Omaha, right there outside your circle.

Another reason, though, is of course, most of that area is barren mountain ranges or badlands ill-suited for farming. To the east there are hundreds of small towns that popped up as local farmers markets, but you need farms to make that happen.

Notable exceptions to the cities-need-ports rule are Las Vegas, which has an industry that doesn’t require the import of export of goods, just people, and thus has a crazy busy set of airports. Phoenix, Dallas, and Albuquerque all sprung up as cattle and sheep towns, but with the advent of the transcontinental railroad were able to become “rail port” cities later on. The same can be said for Atlanta, which has no port but is a state capital, and thus became the local hub for several area railroads, and later became a big city.

Calgary and Edmonton have entered the chat

Omaha is kinda big

It is interesting because Canada has three sizable cities in that comparable region to the north: Calgary Edmonton and Winnipeg.

It’s dry, cold, and winter feels like it lasts dang near forever. The growing season is very short. It’s hard to expand the roads to build homes in the mountains too. The mountainous land is expensive. If you’ve traveled on I-90/94, there’s really not much east of Billings for most people. A few people love it. I don’t get them. I think it’s some of the most desolate land in the country

Because no one wants to live there

nobody wants to live there.