I have never specifically bred for mouth, but there are lines that are known for above-average mouth.

It's just that there are other traits which are so much more important than mouth that, if it becomes a choice of selection, most breeders are going to select for gameness/intelligence/athleticism/air over "mouth" if it's an either/or situation.

Of course mouth is a good thing to have, but there are several traits that are much more important to have, so mouth often gets pushed by the wayside ... because so many individuals are worthy dogs who don't have it.

That said, many of the hardest-mouthed dogs, historically, were linebred/inbred animals (Zebo, Tornado, etc.)

Jack