I don't have any complaints for Raspbian specifically (in fact I really like it on the Pis), it just strikes me as odd for newcomers to go for the niche distros.
... I reckon you're likely to take the path of least resistance and use the one that's right there and you know, rather than start learning something else.
Bear in mind his perspective: he's old, he has limited time left, and he's built not one but two better OSes that should have replaced the first OS he built: Unix.
So he probably is disinclined to spend time or effort on leaning a 3rd party knock-off of his 1970s work.
If you use RasPis, why bother to learn any other knockoff?