It is mostly just supply and demand.
If you search for straight razor on E-bay at any given time, you're gonna get about 900 legitimate auctions that aren't zeepk and stuff. 5 of those might be over 1" wide blades.
Now, some will argue that big blades "Aren't rare" and they can "find them just about anywhere.."
Well, that may be true, and I have no doubt you could find one somewhere if you really went looking...but for every one 8/8" + blade you run across at flea markets, antique shops, etc... you'll run across about 20 or more 7/8 or less blades.