I think the show got progressively worse. The first couple episodes were bad, but at least it had a few witty lines. If they improved upon the timing the show may have actually been funny. But, then the last two episodes didn't even try. The writing just got lazier and lazier as the show progressed, and they never fixed the timing issues.
The series was DOA. There's no point in a critic staying with a series if it barely has a fanbase and is not going to survive the season, especially when there's many other, better shows for them to watch on Sunday. I mean, a critic can choose to write about Desperate Housewives, Breaking Bad, The Amazing Race, Family Guy, The Simpsons or any other show on Sunday that's higher quality and has a larger fanbase than Sit Down, Shut Up. Why would they waste time writing about Sit Down, Shut Up when the quality hasn't improved and the fanbase hasn't grown? It's rather pointless.
By the way, I haven't actually read through all of the reviews, but usually the amount of episodes a critic reviews depends upon the screeners that a network sends. If Fox only sent them one episode before the premiere, they're probably only going to review one episode.