At any given time there are about 150 Maltese listed on Petfinder.org. And any rescue person will tell you that at least half the dogs they take in never need to be listed there as they already have an adoptive parent ready and waiting for them.
Purebred dogs are given up for the same reason mixed breeds are, many because the owners didn't really think the decision through enough and make that 15 year "to death do us part" commitment.
They get jobs that require traveling, go off to college, move to a place where no pets are allowed, have children who have allergies, in short, have lifestyle changes that won't accomodate a dog. Or they didn't realize how expensive that cute little pup would be. (We had one turned into rescue last year at the whopping age of 1&1/2. He needed surgery for luxating patellas on both knees which would run in the thousands so his mom gave him up. I heard she got another one just a few months later...) Behavioral problems like not being housebroken or male marking (again, usually because the owner waited too long to get him neutered) are another reason.
I saw a statistic recently that said 1/3 of dogs who come into shelters are purebred.