[Keith]

Telethons existed to fill time during which people were asked to phone in and pledge money, and to give them plenty of time to do so. A lot of the content of Children in Need in the old days was about that mechanism of raising money - talking to the phone operators, listing which banks to go to, addresses of where to send your money etc.
That's all gone now. It's just text an amount to this number.

Why have a single 8 hours of TV when they can promote that text number during documentaries and other specials over the course of a few weeks? It gets a wider range of viewers. They still get viral clips and talking points, it just doesn't all happen on the same night.

Stand Up To Cancer on Channel 4 does the same thing just over a longer period and arguably a lot better - lots of special entertainment programmes over months

As far as I'm aware the studios at TVC were owned by the BBC, and not its commercial arm. This presumably made it easier and cheaper to give over a studio to Comic Relief for an entire day or two, compared to current arrangements. I'm guessing these days by finishing at 10pm it allows for the set to be dismantled and studio vacated the same night. Modern sets with video walls presumably take more time to dismantle than the wooden/fabric based sets of old.

Also, back in the 80s/90s they had the advantage of most other programmes filming in studios rather than on location, notably with sets in TV Centre. Which presumably made it easier for sketches such as www.youtube.com  to be filmed, especially if talent happened to be already working in another studio for a different programme.