Quite simply, public perception.
Every time a state dinner is held, the tiaras come out on full display, not only for the attending royals and nobles, but for every woman who was invited that possesses one.
To hold a momentous occasion, like a Coronation, and to have only some women in the royal family don a tiara, would leave people wondering why the others didn't. With the most likely question being: Why does King Charles hate his nieces so much that he wouldn't even loan them a tiara for his Coronation?
Not that I would put it like that, but many, many royal watchers would.
So unless BP puts out a statement that no tiaras are to be worn for the Coronation with the exception of working royals, then it would be terribly unseemly for the York girls to be denied the loan of one of the family's many, many tiaras.