I agree with Edd. I think it's more due to how many have unsurfaced since they stopped selling in shops as oposed to how popular the figure was. I remember Klaatu being quite popular when i was a kid. Him and Weequay were 2 figures i asked for for my birthday and i remember being pretty excited about opening them. I do however think a figures popularity now has a lot to do with a) how many are available and b) how expensive or cheap they are. And one can often lead to the other. What i think you will see happen tho, in years to come, is a lot of the less popular figures (especially ROTJ), Gam Guard, Bib Fortuna, Admiral Ackbar and so on, will become rarer and rarer and eventually almost cease to exist due to UGrading. We're already seeing it with mocs like Ackbar and Nien Nunb. Once given no thought whatsoever by collectors and now people are realising they're 2 of the more difficult to find especially in good condition. Gam Guard will be next. How many of them do you see around now-a-days??
I've been searching for Ackbar for 2 years both on trilogo and ROTJ but the only ones that come up are yellow bubbles or shitty cards. The search goes on...