Here's why:
* Breed Standard: The Old English Bulldog breed standard specifically describes the tail as "short, low-set, and screwed." This means it's thick at the base and tapers to a point, with a slight curve or "screw" in it, but not a full curl.
* Genetics: The gene responsible for curly tails is not prevalent in Old English Bulldogs.
While you might see some individual dogs with a slight curl in their tail, it's not a characteristic of the breed as a whole.