From the explanation I had heard, they took Worlds placements into account but also looked at overall success over several years (did their Worlds performance/placement come out of left field when comparing their previous seasons/performances), fan following (what teams are going to sell tickets), current season's potential for success (the season hasn't started yet so we don't know what these teams look like.. did a small team of 18 graduate 7 seniors that could effect their success this year..), etc etc.
Don't know if it's accurate but it made perfect sense at the time, and would make sense why the large senior/coed teams always have the most teams (most popular). I imagine it's not as easy of a process as inviting the top 3 placements each year.