Look the IT Crowd is a fun show, but 24 eps plus a finale 3 years after the fact? That's just not a fantastic output. Comparing it to Sherlock is a bit silly.
Comparing The IT Crowd to an American sitcom is a bit silly. Graham Linehan writes and directs every episode while most US sitcoms have a staff of writers and a pool of directors. Linehan tried bringing in more writers for the planned fifth series but it didn't work out, so he scrapped the series in favour of this special.
While there are advantages to both the UK and US systems of TV production, I far more admire the British tradition of creators "owning" their shows. It generally prevents situations like the Community calamity where Dan Harmon was removed from his own show by the network. That sort of craziness just couldn't happen on a show like The IT Crowd.