Question

Why is Higgins disappointed when the visitor turns out to be the flower girl

Answer 1

Higgins is a linguist - and he wants to study various dialects and accents of the English language. However, when the flower girl, Eliza, turns up at his doorstep, he realizes that he has already studied her accent (Lisson Grove lingo), so he doesn't really need her. This is what he says when he sees her:

HIGGINS [brusquely, recognizing her with unconcealed disappointment,
and at once, baby-like, making an intolerable grievance of it] Why,
this is the girl I jotted down last night. She's no use: I've got all
the records I want of the Lisson Grove lingo; and I'm not going to
waste another cylinder on it. [To the girl] Be off with you: I don't
want you.

However, he changes his mind when she tells him she wants him to teach her how to become a proper lady.