The broader question I've always had, that's not addressed in the article, is "so why do California and Texas build so many stack interchanges while the rest of the country mostly does not?"
> And the design that generally provides the most capacity, on the smallest footprint, (often for the highest cost), is the stack.
This would explain why there are plenty in coastal California, where there are tons of people to move around and where space is very much at a premium.
I've lived in CA and the Northeast. I didn't get the impression that either region uses them preferentially. Certainly I see them more in CA, but I feel like that is probably because the number of interchanges is higher, ha!
Stack interchanges are very common for high-traffic areas, some of which wouldn't necessarily show up on your mental radar.