Why 25 episodes?

They run a show for six weeks and then it stops and then they run another show for six weeks. It works the same way as your American shows that have limited runs (Todd Margaret's third season which recently aired only had 6 episodes, for example).

There are often years between series because they don't run on continuous production cycle like most shows in the US. They'll produce six episodes then, typically, the production will shut down until the creatives are ready to make more. Of course, that's not always the case and some shows do, pretty much, produce their series continuously like in America.

And this was never on the BBC, for the record. It was produced for Channel 4. There are more channels in the UK than the BBC, just as the US has more channels than Fox.


Anyway, it's largely down to the way that things are done. Most scripted British shows are written by the show's creators and, as a result, most scripted British shows only have one or two writers. The infrastructure (not to mention, the budgets) just don't really exist for a British series to be created and then to hire 10-15 writers to each work on an episode more or less concurrently whilst a showrunner oversees everything, which is how it works in America. Basically,
in the UK, one or two writers will write 6 episodes of a show over about a year. In America, 10-15 will write about 1 or 2 episodes of a show (each) over about a year, which adds up to 20+ between them.
Of course, some American shows do follow a similar format and some British shows have used the American format of a writing team (though they've typically been lowest common denominator rubbish akin to a British Big Bang Theory or Two and a Half Men, so you're unlikely to see them any time soon).
As a result of writing more episodes each in most UK productions, there are typically longer gaps between each series (season), because the writers will need more time to recharge. They're also rarely contracted to a production in the same way that they would be in America, so they'll typically explore other projects on the side which is yet another reason why you might get years in-between in series (season).


Now, in the specific case of The IT Crowd, pretty much everybody involved actually wanted to make more (including Channel 4).
But the cast have all become very successful and it became increasingly difficult to get them all together for the duration of an entire series. Chris O'Dowd has become fairly chummy with Judd Apatow and keeps popping up in American shows. Richard Ayoade has a very successful directing career. Catherine Parkinson is doing fairly well for herself on The West End.
Graham Linehan (the writer/director) wanted to make more but just felt like he was out of creative juice. He tried to write a fifth series in an American style (using a writing team) but it proved impossible to organise given that that's just not how TV works here and he eventually gave up on the idea. In the process, he came up with a few ideas that he wanted to explore and, instead of just waiting several more years until he felt ready to write more episodes, he just turned them into the one-off finale special, so that he could give the show some closure before it actually became impossible to re-assemble the cast.

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