The NCAA did it to address two out of three of the biggest concerns of the two divisions.
The biggest concern for D-II is a dwindling membership, consistent with the number of schools that elevated during the 80s and 90s, and an unclear membership philosophy (as in, what *binds* these schools together other than "We want to give scholarships, but not, like, a lot of them).
The biggest concern for D-I has been the opposite: *too many* teams joining D-I with no observable effort in producing a D-I level athletic product, because NCAA hoops money is pretty low hanging fruit.
Putting in specific verbiage that says "Canadian schools can join, if they go through D-II" meets both of those concerns for each division: it gets D-II a ready source of members, and it keeps them from thinking they can go straight to D-I and get hoops money.
Incidentally, D-III's biggest concern is a split between schools who are united by the theoretical common state of "offering no scholarships" but who diverge significantly in their athletic philosophies. You have schools like Mount Union who recruit, and put together top-tier athletic programs without scholarships, and schools like MIT who treat their varsity athletics program as little more than club teams with a different name.