kwaw
.....
No real answers to that question. It may have just been a personal style. It may also be because some members of his patron family:
had faces like that.
Now, Caterina Sforza was much better looking:
Love her eyes. Despite being painted in the half-asleep mode typical of Renaissance women's portraits, she still seems like she had a quite lively intelligence, and was capable of all sorts of things. . .
Ahem. But she still had a rather round face. They may be idealized portraits of the Visconti-Sforzas.