When Pope John Paul wanted to give a speech to the Cardinals reminding them that the cardinalatial red signified a readiness to die for the faith, there was only one Cardinal martyr that he could refer to.
And St. John's martyrdom was all but completed before he was named a cardinal.
In defense of the thousands of non-martyr cardinals, though, by the time the office developed it was quite uncommon for a bishop to be martyred, which isn't altogether a bad thing. One might also suggest some cardinals, such as Cardinal Kung, have been white martyrs, although that doesn't do much to illustrate the cardinalatial red.
And of course the question is never, "How did your team do?" Who can count the number of lay martyrs? Does that make layfolk feel complacent, as the dearth of cardinal martyrs might make cardinals feel uneasy?