In my local shelters, it seems it is mostly lab mixes, pit mixes, or lab/pit mixes... although border collies and aussies mixed with who knows what else aren't too far behind. Some may be bred for because they're pretty or are thought to be combining desireable qualities (i.e., mixing poodle into EVERYTHING to make it a magically hypo-allergenic dog and to be able to create a cutesy name with the word 'poo' in it ), but a lot of it is just whatever dogs happen to be popular among irresponsible owners in that area.