1) They're well designed. Industrial design is important, as the rise of Ultrabooks and Apple has shown.

2) They have a fairly high end but affordable brand name. You either have a coffee maker or you have a Keurig.

3) It makes it convenient to try new flavors. My drip coffee machine won't make me a chai latte or hot cider at the touch of a button. My Keurig can, while also providing me with good coffee from a reusable k-cup.

4) Marketing makes sure they're everywhere and everyone has access to them.

Good variety, virtually no cleanup, no measuring of coffee. Not the cheapest option, or the best tasting coffee, but a great solution for me.