I can't believe Thrawn is last place.
Thrawn is so '90s.
The others (with the possible exception of Kir Kanos) are current or at least more recent characters; thus more popular among newer fans and more "on the radar" for even old-school fans.
Mara Jade: perenially popular due to NJO, etc.
Asajj Ventress: 2003 - present
Padme Amidala: 1999 - present, and the "snow bunny" outfit in question is from 2004
Quinlan Vos: 2000 - present, not to mention a popularity boost from dialogue mention and almost a cameo in ROTS
Kir Kanos: last appearance in 2000-something, right? Anyway, I bet a lot of people are simply voting because they want a Royal Guard, not because of any attachment to the character.
Grand Admiral Thrawn: last novel appearance in 1999--and that's if you count Flim the impostor and that unfinished clone.