"Stock" doesn't mean that the tartan is in stock at the store; it means that it's one of the standard tartans woven by most mills. Tartans which are not stock have to be specially woven and hence cost more. Basically, the mills weave the more common tartans just as a general rule, because they know their suppliers will buy those tartans; special tartans they only weave to order, which both requires extra setup and also negates any economies of scale.