Why does bulldogs skin is hanging?

Answer 1

Because of excess flesh not necessary to cover the face. That excess flesh has nowhere to go except down.

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Well, that answer speaks only to the (literal) gravity of the situation.

If the question is why does a bulldog have so much excessive or extra skin? The answer is they were bred to capture and suffocate bulls and bears. They are a man made, hunting dog of ages past. The excess skin was bred in to make it more difficult for a larger animal's teeth to grab on to them and do damage.

The lower body (from just under the rib cage) is much smaller and was bred that way to allow the dog more protection when being bitten and thrashed about by the likes of a bull or bear.

The excess skin that forms the "jowls" around the mouth, and droops down their face was bred in to them to act as suffocating "baffles" when they placed there entire, extra wide mouth over the snout of a bull or bear. The dog would clamp down on the bulls snout, closing the mouth and covering the bull's (or bear's) nostrils with those floppy jowls. Every time the animal would breath out, the jowls wouldn't allow an inhale and the bulldog would bring them down.

The nose wrinkle was actually breed in to divert their prey's blood from running into their eyes while clamping on the face.

Hundreds of years later, most are like "Winnie the pooh" in temperament. Great with children and as loyal as it gets.