Why do you think there has never been an hour-long sitcom series?

The closest we've ever come to this is with some "dramedies" like *Ally McBeal* or *Glee*, but they're not considered sitcoms.

There have also been hour-long episodes of half-hour series, but these are always "special episodes" and never the norm.

Is it just part of the accepted sitcom format that an episode needs to be a half hour long?