Because Apple was so known for their use of single uniform colors in all their previous aqua-era designs. Yet somehow they were still called simplistic designs. Those stripes in the candy iMacs were completely one toned, right?
Adding style to a simple design does not add any complexity. I honestly love the new design and think it looks more interesting than if it were just a flat color. Not to mention, the build quality looks even a step above the 4S, which is already top notch.
But maybe people should hold off judgement until we've actually seen the final product fully assembled. It's pretty hard to get a grasp on the whole thing when you only look at a single part that is not even confirmed yet.
The Two-Tone look is not added because it should look neat or "stylish". It's added because there are technical difficulties regarding reception if the back is made of metal (see iPad 3G back). So obviously Apple Designers are facing the choice between plastic, metal and glass. Plastic has good reception and is durable, but feels cheap. Metal is durable, feels nice but has bad reception. Glass has good reception, feels nice, but is not very durable.Dieter Rams said:Gutes Design ist so wenig Design wie möglich.
Making glass more durable is not very easy to do; either you make it thicker (generally not wanted) or you change it's chemistry. Apparently Apple was not satisfied with progresses there.
Making plastic feel nicer is also impossible. Just the knowledge that it is plastic makes it feel cheap.
Improving the reception of metal is like asking to change the laws of electrodynamics, so no.
Apple thought the best of all these options is combining plastic and metal in hope that the advantages combine nicely and NOT because it looks cool and adds something interesting to the design. I don't think Jony Ive would ever do something like that. Every detail on his designs has to serve a function. I don't know about the old iMacs though, I've never used them, so no idea why they chose to build them like that.