It's helpful to look at the history of landscape art in its entirety, which is actually a pretty short history. Until late in the 18th century, there really wasn't a tradition of landscape art the way we think of it today. People painted landscapes, but with a few isolated exceptions they were considered background for the real subject, which involved people in some way. The landscape was the stage set on which the action took place.

Landscape became an independent tradition when artists started treating stage set as the subject itself. People may have been present, but the roles were reversed: the people served the landscape (providing scale, or simply as small elements of the scene) rather than vice versa.

Removing people entirely can serve many purposes. For one, this is just the idea of landscape taken to a logical conclusion. The subject is the land, so why do you need anything/anyone else in the frame?

Of course, this is too simplistic to answer the whole question. There's also a technical consideration: in the early days of photography exposures were long and people were difficult to capture. In the first photograph ever printed (a cityscape by Niepce) the only form recognizeable as a person is a shaddowy figure standing still to get his shoes shined.

Another reason to erradicate people has to do with the kinds of wilderness myth building or metaphor making that Ansel popularized. But that's a whole other topic.

Yet another reason is that some pictures represent people in indirrect ways ... and the photographer doesn't want any direct representation of people to overwhelm this.

With respect to Frank's snark about the settlers killing everyone in their path, it's worth noting that the pioneering landscape photographers of the West usually put people in the picture. It's hard to find anything by O'Sullivan that doesn't have some kind of figure in it (occasionally his covered wagon acted as a surrogate). Same with Carleton Watkins. William Henry Jackson more often took unpeopled pictures. These guys may have been party to conquering the West, but they seemed to feel more effective at this when they included some conquerors in the frame.