The answer seems to be two-fold. Despite the knowledge and increased awareness in recent years of exactly what declawing involves, in addition to how a cat's body performs so many of its graceful and often gravity-defying stunts, there still lies within our society massive ignorance on this subject. And, as they say, ignorance is bliss. Allegedly. Having worked with hundreds of cats in rescue work, I've been witness firsthand to the effects of declawing - and I'm talking long-term, often lifelong effects. Call it anecdotal, declare it isn't scientifically-based with oodles of statistical numbers to back it up, but the plain truth is that you are more apt to find behavioral problems with a declawed kitty than with one who is not, all other factors being equal.
The first that comes to my mind, for obvious reasons, is litterbox issues. So very many cats we've worked with were experiencing difficulties with the box, and the one common thread was in the lack of digits. Those who are declawed are so much more likely to end up having box issues, and the kind in fact which are more likely to be unresolvable. Cut off your fingers - see how well you do manipulating your way through the day. And should the surgery not go 100% textbook, the risk of causing permanent, painful damage rises. Some kitties have simply ended up too sore to walk around, or at least to do so without being cranky, biting and swiping, and avoiding the box.
Now, there are many who will counter that they've had their cats declawed, with no ill effects. Obviously, this isn't going to affect every cat, every time and in every way the same. But to me, these folks who make this claim just don't realize they are lucky ones. And their cats are lucky too. Put it this way; if you're looking to add a cat to your home, and want it declawed, just head to your nearest shelter or rescue group. There you will find plenty of declawed kitties, at least some of whom have been given up by their owners because they could no longer live with whatever behavioral effects plagued them (and their cat!) thanks to that surgery. The leather couch is still intact:; the family pet is facing death squad by injection.
With the invention of SoftPaws/SoftClaws, little rubber tips you can fit over your cat's claws to keep them from being scratchy, there just really isn't any excuse for anyone to declaw their cat now. Of course, many still do, and many still will, some placing a preference of material goods over their "beloved" pet and others naively believing they are doing what's best. And there will still be some vets who can't pass up the $$ they see in the eyes of that poor kitty. All of us can do our part, though, to limit that occurrence. Train your own cats to use their scratching posts/pads, trim their claws (more than once a year) and spread the word that declaw is a no-paw. Maybe you'll be letting someone know about it, who just never gave it a thought before. Maybe you'll be securing a good future for a sweet and loving cat - who will stay that way.