Drat. It seems I found this thread way too late.
I know exactly why Brock hasn't fallen for Jessie:
She is too manly for him. Too Tom-boyish. Brock likes girls who are sincere and sensitive, not to mention developed and know how to carry them selves as a "woman of character".
Jessie is over-bearing and very macho, not to mention conceited. She think's too highly of herself and is an attention whore, kind of like a certain red-haired, pig-tailed gym leader he travels with who has a lot of the same qualities as Jessie. (I hope she didn't hear me say that)