I'd imagine that the problem with stocking components and small parts is that they tend to sit on the shelf forever and it costs a pretty penny to get it on the shelf in the first place. Some shops are better at this than others, from what I've seen. But most shops, if it's components like shifters, derailleurs or cranksets, will want to order those on a per customer basis. These are just too expensive to have a lot of them sitting around for years on the shelf.

Plus the internet. Enough said.