Mr Lapenta, where do you get this nonsense from? One reason a number of shops don't carry a big variety is because some of the lines they carry don't allow them to. Some lines even dictate how much space can be given to a competing line. As a dive shop owner I can tell you it is illegal and stupid for a manufacturer to try and tell you what lines you can carry. And trying to tell me how much floor space I can allocate to another line, not unless they are paying my rent.
The reality is like any other business. You look at what manufacturer has to offer and try and find the brands that covers as much as possible. Brand x might have regs, bc, computer, and of course mask, snorkel fins. Brand y might be lacking in one or two of these products.Just like any other business you buy more of a certain brand you get better pricing so stock more of their gear. You sell what the customer needs and hopefully in the end you make enough to pay the bills and bring a few bucks home to the family. Same thing everyone else does when they go to work.