Delta up until about 1984 had roughly 80 nonstop flights out of O'Hare, serving a large array of destinations from Minneapolis to Boston to New Orleans. Many of the routes were inherited in the merger with Chicago & Southern Airlines. They never served international destinations, so Milan was never on the list (perhaps you meant Miami?). 747s were used to Miami in the 1970s.
It's simple why Delta discontinued most of the service - deregulation. They simply could not compete once United and American made O'Hare a major hub. In the early 80s Delta must not have seen the future coming because they built a 12-gate concourse at O'Hare exclusive for themselves. But shortly after it opened they began scaling back and building up Cincinnati instead.