1. Because they're not great cars
2. Because they're too big for some
3. Because they're simply not 'desirable' - that's why they're cheap
4. Because 'I drive a Mondeo' says 'I don't care too much for cars'
5. Because not so long ago both my parents had one...
When I bought my VW Golf TDi, I didn't think 'what am I doing? For the same £4k I could have a bigger, longer, newer Ford' ... I thought 'I quite like VW Golfs, they're well made, the Mk3 I had was reliable, I think I can afford a car which is the size I need, with solid build quality, good residuals, the missis will like it...' etc. etc.
Granted, the Mondeo does exactly what it says on the tin. They make a lot of sense for a lot of people, because they're not a bad car. But everytime someone asks 'what do people think of 'xxx' for my next car, EVERY thread becomes a debate about why the Mondeo, - being cheaper AND bigger, would make a better buy. Well guess what - poeple are prepared to spend more AND get less, if it involves buying the cars they want, not Mondeos. Shocking.