In last year's NCAA men's XC 10k, the top 10 men were separated by 14 seconds. 10th to 20th was 9 seconds. 20th to 50th was 23 seconds.
In the women's 6k, the top 10 were separated by 40 seconds. 10th to 20th woman was 18 seconds, and 20th to 50th was 23 seconds.
6k is already enough time for the top women to separate. If I recall, the race was basically over by 3k. If it were 10k there would be HUGE gaps between the runners.
In the end the distance of an XC race doesn't really matter, it just needs to be short enough to stay interesting but long enough to separate the best from the rest. For women, that distance is shorter.