Why? Well, many rulers have lost their crowned heads in recent centuries!
It seems to me that the Church in this is just going along with the tide of the worldly powers. Catholic triumphalism has long since been greeted with the sword and the sword won the day! So the sword wielding soldiers still dress up but no king, prince or priest will dare to do so!
This is part of the modern hypocrisy of egalitarianism. The ruler dresses humbly yet rules all the more ruthlessly.
So, the slaves dress up and the kings dress down.
All the more reason for low ranking clergy to dress up, especially if they are not interested in becoming princes! But, beware, you may very well become a cassock (or cappa) martyr!
N.B. Even Pope Francis himself never goes out without his cassock, which is proper!