Hmmm, because I think people want instant gratification, and 8-team leagues take less time to fill up than 16-team leagues. Sixteen-team leagues can sometimes take a week or 2 to eventually fill up (because, as I said, people want the process to be quick). I must admit, I sometimes do the same thing... very rarely am I in the first 2-6 people to enter a league, but I will enter it if I know that it is popular (with less teams due to enter, and thus, time). People are drawn to popularity I guess! And they always somehow fill up eventually.
Established 16-team leagues like the League of Gelling and the Gold League will always take less time to fill. But very rarely will I enter Dog Rolling Around in Olive Oil League or Wipe My Arse League unless there is a demand for it (and they always take ages to fill up... I can't think why...)
In other words, I typically tend to enter leagues with a proper name (that are popular).