"Cambrian Explosion" springs to mind.
Javascript libraries, Web frameworks and CMS's all seem to have this theme where the perceived difficulty of rolling your own is less then bending something existing to your will.
They've recently been trying to encourage everyone to make the switch to digitial TVs. I'm sure it's a lot easier to get people to upgrade their browser than buy a new television.
That said BBC produce a lot of content for the web that not everyone can get (people don't all have computers), several digital radio channels (not everyone has digi-radio), several digi-TV channels (a lot can't even get the signal, many don't have the hardware).
So the BBC aren't worried about large scale access issues, why worry so much about obsolete browsers?
Maybe being backwards compatible to IE 5.5 is important enough to switch to this I don't know.