I think Disney might be leaving it as long as they can to get as much out of the first season set as possible. If both were on sale at the same time and in the DVD charts together, some people might be only tempted to buy the one (obviously not true Muppet fans like us tho
) and Disney will likely want to make as much from each season as they can. Once the first season's sales drop significantly, I'm sure we will see the next installment.
Here's hoping anyway.
The Muppets Valentine Show would make a nice bonus too!