Talking on Twitter in February 2014 about why the last episode of each series is a compilation special, Charlie Brooker said:

We do 5 regular eps then a comp ep because that's what we're commissioned to do, btw. So the 'new' stuff is on the house. THAT'S MY SPIN.

We chiefly do the comp for the positive viewer feedback. "@[redacted] fucking hate it when people do that. Lazy. I will not be watching."

To answer another question, we could do a longer run than 5 eps if commissioned to do so, but for a really long run we'd need a bigger team, more contribs etc etc. To produce the material we currently do takes 7 days flat out and often means long nights etc.

You have to factor in time it takes to watch & sift through hours of news footage etc before you can write a section, for instance. Something like Daily Show must have huge team of researchers, writers, producers etc to generate several new mins of material nightly. Our show is v diff beast and is chiefly assembled in edit suites throughout the week. We prune away about 25% of each item as we go. It's a very dense show in other words. To do a *much* longer run we'd prob have to cut back on the number of sections I do per ep. Essentially there's nothing stopping a UK Daily Show, but the Wipe 'format' isn't conducive to that kind of nightly volume...

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