I'm presuming that's not the case any more? I can't really remember the last time we had a programme shown that had been sitting on the shelf for years, but in the early-mid 00s it happened all the time. I remember the first ever show on ITV3 was an episode of Rebus that had been on the shelf for 3 years!
It was a bit different with Rebus, in that it was scheduled to be shown just after 9/11 and was initially dropped for reasons of taste because it was very violent. However the series, of which that was the last part, hadn't done very well and so ITV weren't in any great rush to put it on again (they may have been looking for a convenient excuse to drop it, really). That incarnation of Rebus was then axed (of course, it was then revived with a new lead and a completely different production team) so in the end they decided never to bother, on ITV1 at least.
There was indeed a period around the turn of the century when ITV were very quick to drop underperforming programmes. I remember in September 2000 they had a two-part drama called Where There's Smoke scheduled for a Sunday and Monday night, but a few days before transmission, after all the listings magazines had gone out, they decided it wasn't strong enough for such a prime slot and weak opposition for a new BBC1 drama, so dropped it and parachuted in a new episode of Midsomer Murders instead, unbilled. They eventually showed Where There's Smoke the following summer, in a less exposed slot.
According to wiki and whether that can be believed or not it will be broadcast on the 13th January 2016. The same thing happened last year where the final episode of the 2014 series and compilation show wasn't broadcast until January 2015 and I don't know why they have pulled it.
It's not so much a question of pulling it, the episodes don't run on from each other so they can be shown independently. One obvious reason this time was because Citizen Khan had to start in the week it did because it finished with a Christmas one, and there wasn't enough space to show every episode they recorded. It peps up the schedule a bit to have an extra new show you can pop in somewhere.
Of course game shows in particular, when they used to record in bulk, used to massively over-record and episodes would turn up months or years after the series they were intended for. I've said before they used to show "new" episodes of Big Break in 2000, still with the old BBC logo and 1997 copyright date, and TVS episodes of Catchphrase were still being shown in 1994, because they had a massive backlog. Catchphrase was a great example of how they would hardly ever be a proper "series", they would just record dozens and dozens and they'd be scheduled in dribs and drabs over the next year or so. At this time of year you'd almost always get a few episodes of The Price Is Right or Celebrity Squares that hadn't been shown before. They were excellent fillers.
I've almost certainly said this before but I remember on Christmas Eve 1992 ITV showed an episode of Take Your Pick, about three months after the rest of the series. It actually started with Des doing a bit in vision, saying "Hi, I've just popped in to tell you about the Christmas special of Des O'Connor Tonight, it features X, Y and Z", and then after that little trailer he said "Now here's an episode of Take Your Pick we didn't have chance to show you earlier". Presumably Thames considered it was worth putting that bit on in case viewers got confused as to why it had nothing to do with Christmas. Funny the things, eh? Love to see that again, I remember it vividly.