If you study the situation in IMAC for example, where the majority of flyers are using 50cc, 100cc, 150cc, and larger engines, it appears that there are two fairly obvious situations. The range of 3 blade props is huge in the sizes those engines require. The engine retailers have solid experience and knowledge of which of those 3 blades work best on their engines. After all, those engines are expensive, prices go into the thousands of dollars, and not high retail volume items. The prop manufacturers are the same. So you've got two things, decent selection and good advice for matching prop to engine/plane. That results in fewer props in those guys stockpiles.

Compare that to the glow engine guys and the alky burner boys have basically one mfg of 3 blade props and that company advises matchups from the 70s and whose props few LHS keep on hand.... but plenty of $5 to $10 two blade props. It isn't a financial hardship to have a double handful of those 2bladers.