BBC3 is well known, but its not a popular channel by a long shot. Its mainly for light entertainmnet and comedy but doesn't really draw in big crowds compared to ITV2 and E4. Think the BBC would be reluctant to jiggle their TV schedules to have all 3 shows on BBC1, would upset their viewers too much, especially if they moved their flagship soap opera Eastenders out of its primetime slot at 8pm. That draws in an avergae of 9-11 million viewers each week without fail. Eurovision semi's would be a struggle.
There is a stigma in the UK over the show. For those who watch it (apart from the fans and the gay community) people love to hate it, they'll watch it and get drunk, wont pay too much attention. Ramble on about 'that Europe' and political voting. The semi's just would just not be popular in my opinion for the BBC to take an interest.
If a country in Eurovision on the night is winning, about to win, have a big artist or hosting the show its going to be more popular. 2010 Germany sends Lena, wins, then hosts the show a year later again with Lena. Popularity = Viewership.
UK sends Blue and Ireland send Jedward in 2011. Viewship for both countries goes through the roof. Think the UK beat ITV in that time slot that year. In 2009 Terry Wogan has gone, 2010 we send Josh Dubovie :?, both years are down on viewers. If there was an attraction to the UK wanting to take part except loyality and tradition then fair enough. But for now Eurovision is just ticking along, nothing special, just same old for the majority of the 63 million Britons.