Well, first off, people don't ask for Weavile that frequently, from my experience. But I have seen it often, and I know why. Sneasel needs to level up holding a Razor Claw at night to evolve to Weavile. But getting said Razor Claw is hard; it's only found on wild Jangmo-os, Hakamo-os, and Kommo-os. Jangmo-o has a 5% chance of being found in the wild, and it and Hakamo-o (which it can call for help to) both have a 5% chance of holding the Razor claw. Kommo-o, which can also be called by it, has a 50% chance of holding it, but also has a lower chance of being called. So, with those odds, most people would rather obtain a Weavile through the GTS than waste time trying to get a Razor Claw to level it up with.
Hope I could help!