Because sailing is racing propelled by wind and water only. Pumping means you're being propelled by human strength. Anyone wanting that should take up rowing!
It's an interesting controversy. I've read that pumping is not allowed on small one design classes, just as pinching and footing in a rhythm to gain windward advantage is also not allowed.
I don't really understand how that can be policed. In any case, the article seemed to imply that the rules for A/C were modified to loosen it up for pumping. It seems that pumping enabled the boat to get up on the foils quicker according to the article. I'm not sure why it would have anything to do with boat speed once the boats were up on foils and it doesn't sound like there would be anything to gain by pumping once the boat is up to speed.
Pumping is a legitimate way to get sailboards up on plane earlier.
Besides, all of the sailors train for brute strength and speed, so why is it illegitimate to use that physical strength for pumping? How is it any different than grinding faster or getting better leverage on the rail?
It seems to me that the issue that traditional sailors have most against it is that it goes against tradition. It's the same as the traditional cross country skiers used to argue against skating in cross country races. It used to be acceptable just to take a few skating steps to get up to speed to go into the traditional stride. But then, the racers figured out that they could change the design of their skis and boots and actually skate faster than stride. Once they broke out of tradition, the argument against it went by the wayside.
So it would seem to be the same thing in sailing. The object is to sail your boat faster than your competitor and you invest all kinds of training and technology toward that end. It seems silly to say ... nope, you can spend a fortune to develop your boat and sails and outspend your competitor, but you can't pump the sails to make your boat go faster. If it is a physical activity rather than a mechanical activity, it would seem to be an athletic endeavor that should be acceptable.