To the problem of them having limited lenses in stock - Shops only have so much storage space, so they only stock what they can shift quickly. If they get a lens in and it sits in the back of the shop for a year, that's both wasted storage space, and money locked up in a product instead of being used to buy new stock etc. Cameras only make up a small percentage of their sales, and very expensive camera lenses make up an even smaller amount of their sales.

So they stock the more common things, and order in the others when they're needed. It's a perfectly reasonable business model, that almost all shops employ. Lets face it, why would you stock something you sell maybe one of a year, when you can stock things you sell 2-3 times every month? Supply and demand.

Source: Family owns a shop which I help administrate. The area we work in has literally tens of thousands of different products, we only have space for a few thousand. So we stock the ones that shift quickly, and order in less common items when customers ask for them. This saves us spending thousands of pounds more on more storage space and more stock, hence helping keep the prices as low as possible by reducing costs and allowing us to put discounts on the items.