Why is it difficult to create a vaccine against cholera

A vaccine against cholera has already been created. It was first made during the 1800's. Even so, it's limited supply for places such has Haiti.

Spanish physician Jaume Ferran i Clua developed a cholera inoculation in 1885, the first to immunize humans against a bacterial disease. Russian-Jewish bacteriologist Waldemar Haffkine developed the first cholera vaccine in July 1892. It is already discovered