This isn't a topic limited to men's soccer, but it certainly affects men's soccer. I knew that there weren't many colleges participating in D3 in the western half of the U.S., but this map posted by NCAA Research really drove home the limited reach of D3 sports. It's largely located in the east and mostly the north eastern quadrant of the U.S. with a smattering of schools in the southeast and Texas and only a few clustered around Los Angeles and Portland.

Why is that? A couple of possible background reasons:
- Fewer schools overall in the west and fewer small liberal arts colleges, which are the traditional homes of D3 sports. Most of the west has large land grant state colleges, while the small colleges dotting the east and upper midwest were often started by religious groups etc before much of the population shifted west and south.
- Path dependence - because there are fewer D3 schools, other small schools don't have any travel partners nearby and therefore D1, D2 or NAIA may provide more opportunities
This can't be the whole answer, though. Although there are fewer schools in the west, there are many more schools in the west sponsoring sports in NAIA than in D3. For example, there are 19 NAIA schools in California, including a UC state school (UC Merced), 15 in Kansas, 13 in Texas, 8 in Nebraska, 8 in Oklahoma, 7 in Oregon, 6 in Montana, 5 in Arizona, 4 in South Dakota, 4 in North Dakota, 3 in Washington. Why did they all go NAIA and not D3?
Is there anything about D3 and its rules, though, that makes it unattractive to, or difficult to obtain by, schools in the west?