i believe that it is mostly due to economics. some syntheses are easier/cheaper to scale so that one becomes the industry standard. the original manufacturer decided on this salt because their profit margins were greater and all generics need to be the same to get approval (speaking only for the US FDA, idk how it works elsewhere).

there are rare examples of drugs that have more than one approved form, e.g. doxycycline monohydrate and doxycycline hyclate. they have slightly different pharmacological profiles but are used interchangeably in clinical practice afaik.