If the chemist adds 1 equivalent of HCl,you get the mono HCl salt.If he adds two,you'll get the di-HCl salt.More than two eq's still produces the dihydrochloride but if his skills are lousy and adds 1.2764eq HCl oh well you'll have a mixture.
So making the 2HCl salt is on the safe side for having a homogen and well defined product,though dihydrochlorides are often very hygroscopic and difficult to handle.
Plus,it happens that the monohydrochlorides are crystallising faster/easier and form more stable polymorphs,meaning they are targeted in the first place.