When Paul Vi was crowned, it was with a tiara bought by the people of Milan, the diocese he had led before becoming Pope. During Vatican II, after hearing of the poverty in S America and Africa, he gave the tiara to feed the poor.
When John Paul I was elected, he was not crowned, but simply installed as bishop of Rome. He still used the tiara in his coat of arms.
John Paul II continued that practice, though there are a few instances of a coat of arms with a mitre instead of a tiara.
Benedict XVI’s coat of arms has a mitre and the pallium, signs of episcopal office and never had a tiara.
Francis followed Benedict’s lead.
The Holy See still uses the tiara on its flag, presumably because there is still a temporal component to its existence. While the Popes emphasize their spiritual authority by using a mitre, there is still a temporal jurisdiction symbolized by the tiara.
To be clear though, the tiara was to conform the papacy to the ways of the world, not the mitre. As crowns and kings became less significant in world politics, this particular adaptation to secular values has become less desirable.