I think it's a combination of many factors, with the number of leagues low on the list: The economy, time pressures on some owners affected by the economy, players signing up then playing almost no live games, frustration over frequent crashes or system down time, a handful of owners who only play in their own leagues, owners signing up for leagues then vanishing when it's time to play, etc.
Staggering the posting of leagues only helps if some of them fill. I've got five leagues going, but the original posting of them was spread out, generally coinciding with the end of a previous season. Unless we want owners to give up on ideas that stall, this is going to happen with the limited # of players who sign up for more than a couple of leagues.
• 1922-39 draft league, 21 in, posted 5/7 (fifth in series of draft leagues; one previous installment took 7 months to fill)
• Blacklist League 6, 21 in, posted 6/16
• 2-Franchise League, full, posted 8/27
• USA Federation Season 15, 22 in, posted 9/19 (this one puzzles me; Season 13 took weeks to fill, so I posted S14 well before the end of that season and of course it filled in 24 hours, without anyone taking a second team. Naturally, this time around it has stalled for a month).
• willsauve's AAA Franchise League, full, posted 8/5 by willsauve, who asked me to run it when he became too busy to do so.
All are continuations of ongoing leagues, so it would seem unfair to owners who have played in the past for me to bail on them.