Another factor is that a lot go into the business because they like diving. Not because they see a great location, love running a business, and have a good business plan and see how to make a living wage out of it. Many years ago a friend of mine went from working for a large national gas company to running a gas station in Chicago that was very successful. He said most stations did not do well because they were not trained business men. His rules were simple. You did not stock anything that would not sell within a week. You did not hire any help, including your best friends son, unless you really need help. You did not always do what the company wanted unless it was good for you too. (Like they wanted him to drop his prices, but not what he was paying for his gas inventory) Etc.
What I hear in this thread is a lot of folks who want the LDS to have an impossible business model. Charge low prices. Carry a big inventory of everything they might want one day, always have knowledgable staff ready to quickly help them., etc. Just one thing that they are overlooking is that many businesses, including LDS, operate at least partly on loans. Some of that inventory is bought on loans of some form. That means that a piece of inventory that sits there for 4 months before it is bought is not just using space, but also costing them money.
Also, the economics is such that most LDS cannot afford a store in a good location so there is very little walk by business. You have to be looking for them. My area had 5 LDS. All but one were in what I would consider out of the way, little struggling areas. The one exception appeared to be doing it all right, with a very active dive club, etc. but then when the owner suddenly died, it closed because it turned out the finances were not going well.
As for the comment about branching out what you sell, it has to also be something that can survive in a suboptimal location. If I own a big store that sells boating equipment, dive equipment, fishing equipment, and I see that I am getting much less return on my dive equipment investment, guess what area shrinks?