I would love it if Jaheira was a romance option so I could romance her in two games, but I understand why they might not choose to do that. It may simply be that the role they envisioned for her in the game is one that doesn't lend itself to a romance--if she is written as a parental figure (attempting to be an omnipresent authority figure) it could be a really bad idea to romance her.
She was already a parental figure in BG2 and it didn't really stop anyone...
