The problem is we are sore losers!
We expect to win because we are GREAT Britain. We have a certain arrogance.
We are not owed anything. We think we are.
The scoring system means unless you are constantly in the top ten, your score might not reasonably reflect the merit of the song. A 14th best song can easily end up 25th. There isn't much room for marmite songs. It has to be a song that has universal or wide appeal.
Plus we are culturally more different than the rest of Europe. Thats neither a good nor bad thing. It is what it is, sometimes an asset sometimes a curse.
We tend to see Eurovision as a bit of a joke. We have a certain snobbery where we only like the upbeat song (preferably so cheesy that its mature cheddar) everyone can sing along with or the quirky song. We don't ever see it particularly seriously. You limit yourself if you take that route as its incredibly hard to do well, without being totally naff. We keep ending up naff.
In the last two decades the more mature grown up and at times moody songs have done well. This is perhaps easier to do. And ironically its what our professional artists do very well. Its just not we see as 'eurovisiony'. So it doesn't get put forward to the UK to vote for and even if it did we wouldn't choose it.
And we've made some truly dreadful decisions too. Engelbert Humperdink? What in Gods name were we thinking? Who watches Eurovision and votes? Should we have won with that pile of shite? Did we deserve anything more than last?
Honestly I think our expectations do not match reality. A lot of that is to do with the two world wars, one world cup mentality mixed with ideas of self importance born of Empire. Where else have we seen that?