How became a hat a must-have fashion item?
Hats were already worn by the Greeks (and even before them).
A Greek painting with a hat : Source - Wikipedia
In the late 16th. century women started to wear a hat.
The best hat makers were then from Milan in Italy and
that's where the term: 'Millener'
In the 19th. century the hats became larger with ribbons, flowers, feathers etc.
Many (young) royals, I think spontaneously about the Duchess of Cambridge,
wear beautiful hats.
Hatmakers really became very popular (Fabienne Delvigne, Philip Treacy, ...)
The royals understand the importance of the symbolic of hats
(a hat can replace a crown).
Hats are even more!
They are the fashion item to show a social status.
Isn't that amazing!?!