This is common sense to me, why is it not to other people?
(I'm not trying to be rude here)
Because not everyone thinks the same way. As I said in another thread, there were people on the left who were totally shocked by the conservative victory (and majority) in 1987. They mix in circles with other left leaning people and think that because so many people they know think this way, the whole country thinks this way. They thought that it was impossible for mrs thatcher to win, given how she was so disliked in some parts of the country.
The problem is that you think 'How can the conservatives win' for the reasons you stated above and presume that everyone is going to think in the same way. They don't.
An example of this is my brother who is absolutely convinced that the lib dems will lose pretty much all of their seats (including sheffield hallam, where he lives) because he can't find a single person who supports the lib dems. He was absolutely shocked when I told him that in some areas (e.g. southport), the lib dems gained seats in the 2012 council elections off the conservatives. He didn't understand how it was possible.
A conservative who has always lived in labour areas.