Economies of scale, plain and simple.
O gauge is a much smaller consumer market compared to HO or N, has been for decades. The ability to do more railroading within a given space compared to O is the biggest factor. That's why HO overtook O in sales starting back in the 1950s.
So with a smaller market, you're going to have fewer (mainstream) manufacturers, and as such, a smaller product selection. Opposite tends to be true if the market is larger. With a larger market, there's more room for a manufacturer to play while making a profit and staying competitive.
There will always be "niche within a niche" vendors regardless of which scale you're in that will also sell specialty items ranging from OEM or reproduction parts, detail parts, building kits, scenery, etc.
