Because if you look at the percentage of surrendered cats with either litterbox or biting issues in declawed cats vs. non-declawed cats, you'll see a staggering trend of declawed cats with those issues whereas non-declawed cats don't have it as often.

Also, declaw is actually the amputation of toes. Not just removing nails - its removing whole toes.

Edit: my rescue is completely against declaw. If we find out any of our adopters declawed their cats, we have the right to go in and remove the cat from the home. Its written in the contract. Though thats another whole issue and I think its downright silly to go reclaim a cat that is living happily with its owners who love it, to return it to a crowded shelter.