It's got a lot to do with coaching and interest, as well as size of the school.
Good coaching will generate interest. Athletes that would otherwise be playing soccer, baseball, tennis, basketball, curling, or something else will run if they can find success. But those guys and gals will stick with the team sports unless there is a good program where they can be successful.
When I came through HS we had a decent coach. He generated interest and we had some good athletes take up running. I was a baseball guy that got converted to running after I won a mile in PE...
With decent coaching the program got better - we made states in xc, almost won a track title (second). At one point, had 6+ kids running sub-5:00 and a couple more really close (not bad for the late 1990's). That coach left for another state and the program declined. I don't think anyone has broken 5:00 at my old HS for many years. It's sad because this is a really big high school.
I think (guessing of course) that small schools have a harder time because good athletes have a lot more opportunities with traditional team sports.
I agree with folks above that there are lots of students that have the talent (i.e. the 5:14 basketball/tennis dude) to break 5:00, and most that could do it with little training never do.