The CNC milling machines that have been around for decades are vastly superior to 3D printers in every imaginable way except one: marketing.
Seriously, what is with 3D printers being pushed so hard by the pseudo techies? The ones affordable to the general public are limited to weak plastic prototypes. For the same entry level investment, you could have a CNC mill that can work with aaluminum. If you look into the used market, a new world of possibility is open to you.
3D printing is a gimmick and will be so until they can work with metal and come in at under $1 million.
You overselling it dude. CNC milling is a subtractive technology and limited to mostly two dimensions. It needs drill heads, if metal, cooling systems, etc. Great for a big shop but not that great for your office. 3D printers are additive and can create full 3D objects with objects inside objects complete with detail overhangs. It's an apples and oranges comparison at best.
3D printers are more popular for several reasons. A milling machine is loud, cumbersome, requires at least some technical knowledge and assembly capability, and metal is relatively expensive. For your average do-it-yourself person who works a full time job, has kids, and a home enviornment where there are extremely limited hours (and space) for such equipment, this becomes an instant no-go.
Compare that to the promise (false as it may be) that you can go buy a 3D printer and some filament, download a blueprint, extrude a name or some simple design on it to make it your own, and press print, and have the printer quietly work on it overnight to have a product ready by morning. Obviously reality is never quite that simple, and things never quite work as advertized, but even with the hang-ups of 3D printing it is still going to typically be a simpler process with more flexibility than metal or wood-working equipment.
But the biggest thing that makes 3D printing more popular than CNC are the types of things being made. Custom light-switch plates for the kids. Cell phone cases, accessories, and other gadgets. Replacement gears and parts for small electronics which were plastic in the first place. Creative novelty and gag gifts for friends and family. RPG figurines for your nerd night out. Sure, metal would be more durable, but plastic (even cheap plastic) is going to be more than good enough for many of these applications, and in a lot of cases the preferred material.
Don't get me wrong, CNC is pretty cool stuff. If I ever have the spare time and money to burn later in life then it is something that I would personally love to at least play with. But for my life right now, as it is with most people, the 3D printer is a much easier point of entry, which will grow into a complete set of CNC and woodworking tools as time goes on.
You have no idea what you are talking about if you think milling machines are limited to two dimensions. In fact, I can't think of a CNC router that has less than 4 axes that isn't part of a specialized assembly process.
As to your other point that milling machines can't produce 3 dimensional obects inside other 3 dimebsional objects, engineers have come up with a miraculous solution to that problem: use two pieces.
I think you are the one over selling working with plastic.
Grandmastersexsay, I understand what you say, but I would say that 3D printing is as revolutionary as CNC was when it first came out. The thing that makes it interesting is the ease; you don't need a workshop, dedicated space or other hardware. And that makes it easier to get into than CNC, especially for education; you could have a 3D printer in a schoolroom, but not a CNC.
Of course, it has limitations, especially with the rather primitive state of the art at present. But I think that anything that gets more people into creating stuff themselves is a good thing.