Keep something in mind: SST films their new seasons a year or two in advance prior to their actual broadcasts, and most movies take about the same amount of time to make, so what we're seeing now was pretty much made around the same time MMW was in production, which would explain Steve's absense, does this mean he's gone from the show for good? We don't know just yet, so we really can't assume anything until we know officially otherwise.
Besides, this isn't the first time a character had to be temporarily recast because Steve wasn't available: Johnny T. played Ernie during Season Two of PWMS, and of course there's the infamous Artie Esposito performance of Kermit.
Keep in mind too, these performers work in a similar manner as freelancing, so a lot of it has to do with availability and scheduling.