I don’t typically answer “Because Hashem Says so,” to most “why”s. In this case, it is poshut in the Torah, that a baal mum cannot “duchen” before the olam. Hashem didn’t give explanations (perhaps it would be distracting to see a Kohein with a deformity, for some reason, and their kavana would be different, or even the Kohanim themselves would be distracted from the kedusha of blessing Am Yisroel). I don’t know what the reason is.
Neither do I really know the reason why as a bas kohein, when after 120 years (G-d willing), I am nifteres, my brothers cannot attend my levaya, should I predecease them, simply because I am married. But that is the Halacha. And therefore, likewise the answer is, “Because Hashem Said so.” There are many things that get an answer like that. We have to do them, whether or not we understand the reasons, whether or not we LIKE the reasons, and whether or not we think we are good Jews without doing those things. It is simply because He Said so. Clearly there is an inyan of kedusha involved.