Because Apple never made it clear from the beginning of rumours of a 5c what the "c" stood for. We all knew they were going to China and the phone was going to be in a polycarbonate body. Put these two together and people expected a cheaper phone. The media expected a cheaper phone, but Apple didn't deny or confirm what the "c" stood for until launch.
People were saying;
C is for Cheap
C is for China
C is for Colour
A classic example is the 5s. What does the "s" stand for? When the 4s was released, it came with a voice assistant called Siri, so people said the s stood for Siri. Some say the "s" means 2nd Generation of the same phone. Some say it stands for a speed bump. All can be seen as valid and true. There is even a crowd of people that make jokes that the "s" stands for SAME.
I could, in a way, say that Apple really has no-one to blame but themselves. Their products are popular and the leaks and speculation run riot with news of a new iProduct all the time. When you have various paper reports and analysts all 2nd guessing them, it's better for Apple to just come out and say, "You are all wrong."
To be honest, The iPhone 5c is a glorified iPhone 5 in a coloured shell. You could easily have bought a iPhone 5 and stuck it in a slim cover with whatever colour or design you want for less of a price. In my mind the iPhone 5c is overpriced and my guess is Apple probably thought they would sell by the bucket loads. The problem for them though is that prices outside of the USA for the phone were just too high on the 5c which made the purchase of a 5s a no brainer. If you don't like the look of the cold steel finish, then buy one of the many millions of covers that are available for you on the market. If you really want a 5c then choice is a wonderful thing
Like others have said also on here, a Black iPhone 5c would of been a good move for them.