Let's face it. All "desktop computers" are boring these days. They are leftovers from when the various parts inside - CPU, memory, spinning hard disk, power supply, etc. - were so big that you needed a huge box under your desk.
Now that everything is much smaller, the whole idea of going to a desk, sitting down and "using the computer" is as quaint as going to the kitchen to use the phone hanging on the wall.
Mobile (phone, iPad) - or at least, easily movable like a laptop - is what people want now. Apple is not going spend much time updating what is essentially an antique for the few people who still want it.
Because you can take a laptop, plop it on a desk, plug in a mouse/keyboard and any size monitor you want for the complete "desktop experience". You can also take it with you when traveling. You can't take your iMac to the coffee shop.