No cite but I heard it was due to the strain on the heart and the circulatory system, that it had to work harder if you’re really tall. I’d link the SD article on the subject but it seems to be about taller than average people, not really really tall people, and doesn’t answer the question anyway.
It’ll be interesting to see how long all of these NBA seven footers that have come up in the last half century live- one of the first after Mikan, Wilt Chamberlain died at 65 due to heart problems.