It is common in wyandottes to get birds with single combs. That's from hatcheries as well as reputable breeders.
There is a link to lower fertility in rose comb breeds so a lot of breeders breed single comb birds in their breeding stock. Wyandotte breeders are the worst at doing so.
Single is recessive to rose so breeders can use the single comb birds because when bred to a pure for rose comb the single comb gene will stay hidden and that's their way around the lower fertility.
I myself hate the idea of doing it but a lot of Wyandotte breeders do it.