One also has to give consideration to the notion that the college of cardinals, or at least those eligible to vote, has to be limited to the number who can fit in the Sistine Chapel. As silly as that sounds, that is in fact the main limiting consideration. So every decision to make a cardinal is taken very carefully. In the US, only New York, Philadelphia, Chicago, Boston, Washington, and Los Angeles are guaranteed to have a cardinal. Washington is smaller, but is of course the capital. The rationale for Boston when Baltimore does not always have one escapes me. (There are in fact more American cardinals, of course, but where they are is not always the same.)
If I am not mistaken, Avery Dulles was appointed when he was already too old to vote for the pope, so it was something of an honorary thing, I assume to set an example in favor of orthodoxy. Theologians of greater world renown such as Karl Rahner would have had no chance.