In the past, date on waitlist and tier were main factors. Finnair has added a lot of factors to this and explicitly said they will never show or reveal how prioritization is done and that there is going to be chance involved. In a webinar 5 months ago (https://www.flyertalk.com/forum/37401244-post4607.html) they said tier is a major factor, and date of booking is used as one factor between waitlist entries of the same tier.