Because Apple did nothing to quash rumors of the iPhone 5. This makes me think that the decision not to launch it was made very late. Usually Apple is better about managing expectations.
In the end, you have to evaluate the device on its own merits. The iPhone 4 may be Apple's most successful product ever, so perhaps this is just "if it ain't broke"-ism. The iPhone 4 is holding its own against all the Android competitors; is it the C.W. now that the 4S will do worse?