My guess is cost reduction. Looking at the application chart, the KL2610 only fits the 2610 tractor, while the KL4030 fits all the CK10 series tractors while providing much more lifting capacity.
Good luck with that one. I tried to get the new loader when I bought mine new. My dealer asked his rep if he could do that, they told him it would void the warranty. So I bought the new tractor, two weeks later, they started coming with the new loader. Was given the run around after that.
I have not been impressed with my 4030 loader so far, lots of flex, noise, doesn't seem to have the same curl as my 2510's loader.
Perhaps the 66" bucket is not a good match for it.