It is hard to find people that use those skins.
-Resource Packs may become incompatible dependant on Minecraft
-As the resource pack uses a single folder can be messy and hard to find a skin
-Dependant on other resource packs it can take a while to load them
-If playing on multiplayer it would mean making sure that everyone has them
-Where as just the URL would be that and it alone.
Not that it can't be done with a resource pack, I just think people may find it easier to utilize