Not all single camera comedies forego laugh tracks: just look at practically every single sitcom produced between the years 1957 and 1971.
The Muppets wouldn't work in a multi-camera audience setup: puppetry can be really technical, it can take multiple takes for us just to watch a rod-hand character grab and hold an object, or seeing a character sitting atop of something then get up from where they're sitting and move on. Audiences aren't going to want to sit through all the different takes and setups it would take for the characters to work within their settings, it would disrupt the flow of the performances for the audience, since most multi-camera audience shows are filmed/taped almost like a stage play from beginning to end, whereas single camera shows with laugh tracks offer producers the ease of being able to shoot different takes throughout the week of production.
That said, I too am tired of all these ghastly laugh-free "sitcoms", you can't even tell they're sitcoms, and while the mockumentary style works well for film (Christopher Guest films, basically), it feels odd and out of place for weekly comedy series.