Because thinner is better!
Or something like that.
I still chuckle at how careful the photos of the new iMac are to avoid that it's only thin at the edge. There's a ugly bulge on the back that kind of makes the idea that this is a thin computer silly.
If they really wanted the display to be thin, then put the computer into the stand and just let the thin screen float in air. Like the old swing arm lamp iMac.
Or better yet, just leave it the H*LL alone. This is a desktop computer that you pick up once, set on the desk, and look at from the front for the rest of its life. They had already achieved a pretty amazing design with the old one. I'm not saying you don't tweak it. Play with the front to make it more amazing. Tinker with the display (sort of like they've done with this one) to reduce glare, make it more sharp and bright. But there was no functional benefit, and very, very little cosmetic impact to making this thing thinner.
And I'm not sure Tim's head should roll for this one, but he certainly should be uncomfortable in his chair. Is it his fault that a welding process is taking longer than they thought it would? Not really. But as a CEO of a major company your job is to weigh the big picture issues, try to predict how they'll come together and make tactical and strategic decisions that are good for your company.
In this case, a good CEO would have weighed the fact that people love the design of the current product, (but just wanted the specs bumped) the impact of lost functionality from the redesign, and the fact that any new manufacturing process is going to inevitably have some glitches, then make the decision about whether to A. Move forward with the new design, and B. Decide when to stop making the old model and announce the new one.
Not trying to be a Monday morning quarterback, but in the same position I would have done a couple of things going into the holiday sales season:
First, I would have very critically weighed the benefit/user experience impact of changing this design just to make the edges thinner. A lot of people have focused on the fact that the DVD drive is gone. Important, yes, but just as important is the fact that this new design required them to move the SD slot to the back. One of the nice features of my iMac is the ability to come in from shooting pics, pull the SD card out of my camera, and slide it into my Mac to upload into iPhoto. With the new design I have to either move my Mac, or stretch over the top of it every time I want to slide the card into it. Kludgy and degraded user experience, IMO.
Second, I would have seriously questioned the wisdom of taking the old iMac off the market before we absolutely knew the updated one would have been ready for the holiday season.
As CEO, does Cook have his hand in every step and process in getting this thing to market? No. But he is ultimately responsible for the strategic decisions that put them in this position. That's why he makes the really, really big bucks. If you are going to get the reward of this type of position, you certainly should be willing to be held accountable for the results of your mistakes. For multi-millions of dollars per year you should be an oracle.