I can't speak for every League, but it has been my experience in my home League and a few Pogo Leagues, that many TDs are already hosting 8 or more hours on Marathon Day. I am sure most of these TDs are very exhausted and would not have the stamina to do that two days in a row.
Also, we have over 30 or more Active Leagues in Eliters. We do our best to not host Marathon Events on the same day as our sister leagues or another league where most of our Players also play. Leagues can only host Marathons on Fridays, Saturdays, and Sundays -- and many prefer Saturdays and Sundays -- so if every League hosted a 2 day Marathon, it would be very difficult to avoid conflicts with other Leagues.
I also wonder how many Players would turn out for a 2 day Marathon. I think most Players are also tired out from 1 day Marathons in the Leagues where they play.
Just my 2 YEPs worth.