The way I see it: Half the problem is that your page might not be relevant (according to google). They want to make sure your ad matches the page it lands on so you don't find all the keywords with no bids and bid on them. Imagine your site is about Lions, your landing page is about water purification and your ad says "Check out these cheap bicycles!" and the keyword you are bidding on is free wallpaper. So to keep people out of uncompetitive high volume keyword markets, they raise your min bid to the point where you wouldn't want to bid on it. Since they can't have humans check every single ad they do it in an automated way and call it a quality score. They probably use very similar algo as they use to rank SERPS, taking into account elements like title, and links, keyword density, etc. The other half of the problem is google is greedy and finds any way they can to constantly raise your mid bids.