I don't think Henson owes us an explanation. But Disney does.
Seriously, though, this kind of deserves an explanation. I mean, when Eric took over Frank's characters, we kind of expected something like that because Frank was phasing himself out anyway. But Steve's been so busy with the Muppets, it's a complete shock to see someone else fill in like that. It would be nice to know just what's going on.
I don't think anyone's demanding that intimate details of Steve's life be revealed, not that that was your suggestion. But I'm sure pretty much everyone is concerned about Steve. It wouldn't take much of an explanation at all. "Steve is fine. Wasn't able to make the show. He'll be back soon." That wouldn't kill anyone to say, would it? ... I just said it and I'm fine. Unless that can't be said, which is why people are freaking out.