The desktop form factor is declining significantly in popularity in the mainstream market in favor of laptops and mobile devices. The reason is obvious: laptops and mobiles are portable, and the average user (and even many pro users) do not need that much power in their local device.

It's still very popular in the gamer and professional workstation market, and likely will be for a long time. It's just hard to cram the processing power that gamers, hard-core developers, CAD and simulation users, etc. need into something as small as a laptop without creating what's comically referred to as a "ball burner" or "weenie roaster." I'm sure there's a female equivalent expression but it's even less polite. :P

Intel has many-core options for these machines, but they tend to lag a little behind. If you have deeper pockets and really want power you can always put a high-end GPU in a server board with 2-4 Xeons and put it in a tower case. Now you have a data center node with a 4K monitor on it.