Basically, the PH has sold in the neighborhood of 5 times a regular D&D product.
A regular D&D product sells about 8-10 times what a d20 product would sell. So, if WotC put out a product and sold 100,000 copies, a d20 company with good distribution and a company with some name recognition might sell 10,000 copies of the exact same book if they put it out instead.