BBC1 has gained a default audience because as more channels arrive, the viewers who watch commercial channels thins out more (and ITV have accelerated this for the main channel in the last couple of years). BBC1 stay strong because there main demographic is an older audience who are unlikely to watch many multi-channels (bar ITV3 and Drama). My grandparents do this.
I'm not critising this gain for BBC1 because they have not pissed about taking audience for granted like ITV.
But the fact does remain, being the biggest channel and the only channel without adverts, BBC1 does have the biggest default audience. As many on here have been discussing, when TVUK moves to ITV, the ratings will drop from what they were on BBC1.
Bytw, I'm not saying ITV hasn't got a feault audience, compared with Ch4. They do. But BBC1 has the biggest because their skew of viewers is older than other channels.
I only base this on what I know, and most of this is exactly what my grandparents do (even though they do watch ITV, its mostly dramas - anything else isn't usually interesting enough). If there is little on offer from the main channels, they stay on BBC1 because that is the channel which mostly appeals to their age demographic.
I'm not critisising this demographic btw, before anyone thinks I'm being ruse or insulting them.