The Dura Ace Allez is less then 4000. The Dura Ace Tarmac is 5500. There's your reason. Some people may choose a DA Allez over, say, a Tarmac Ultegra. Just like in mountain bikes with hardtail vs. Full suspension, more or less. Sometimes you just want the better components.
Of course, and Dura Ace or even Ultegra bike is far out of my reach right now anyways, so I'm just speculating.