Because pistol grip stocks are nontraditional looking compared to the late-1800's aesthetic of most wooden-stocked hunting rifles, and many anti-gun extremists consider them Scary Looking. Which is enough to get them banned...
Another factor (and perhaps the main reason PG stocks were a target) is that the prohibitionists want to ban ALL self-loaders, but realized early on that that would be political suicide. So they concocted a features list that would exempt many traditional-looking self-loaders like the mini-14 and Remington 7400, while grabbing as many of the others as they could.