Yes, but Harry Potter has a huge international audience right now, and not just kids, either. My sister, in her 60s, is a major fan of the Potter books, and so are most of the other people of all ages that I know. King Kong, on the other hand, is not exactly a new franchise, and it hasn't been rejuvenated lately.
The Harry Potter play also benefits from not being a musical--that is, they haven't tried to pull off something that is beyond the range of the creative people involved. As musicals go, King Kong is awful, and so was Spiderman before it. Both belong in theme parks--if anywhere--but are just not good enough to profit on Broadway with those ridiculously outsized production costs. They'd have had to run decades, and they're just not good enough.