I'd have to say that I find
Jamison to be a more appealing alternative for a girl than Jamesine or
Jamesina because to me they're both just
incredibly ugly. I usually like female names that are male names feminized with an -ine ending, but not at all with those. I know there's a lot of debate on the board about the -son ending and if it's only appropriate for one sex, but given how popular and largely accepted -son names are for girls nowadays I'd much rather go with the name that was actually aesthetically pleasing rather than the one that I think sounds more like a playground taunt to an effeminate boy, which is just about how I'd describe
Jamesina.
Not to say there's anything wrong with your opinion or your suggestion, but
that very well could be the "why not" behind Jamesine/Jamesina. It's like
Madison and
Maude; I don't care much for
Madison, but I find
Maude to be incurably ugly and would greatly prefer going for the -son name instead based on the aesthetic gap between them.