Because properly set up drop bars are, by no small measure, the most comfortable.

At least for me.

All straight, or even straight-ish, bars irredeemably suck. One position, which not only catches wind like a clipper ship, but somehow manages to make my palms, wrists, elbows, shoulders, and back all hurt about the same amount. Bar ends add one more position... which is pretty much always wrong.

Weird German "trekking" bars have all of the downsides of straight bars with bar ends, plus the additional disadvantages of having another straight bar clamped a few inches farther away from you, giving you an even-less-comfortable version of the position that you hated on the straight bar you're replacing. Oh, and you can't use a handlebar bag.

Moustache bars: Stabbing pains in both hands before I reached the stop sign at the end of my street. Other than that, I liked them quite a bit... because of the ways in which they were like drop bars.

3-speed style, North Road, Albatross, etc... Pretty good, on the right bike, ridden the right way. These are the hardest to get right, IMHO, because there are so many different sizes and bends, and no real way to know what you're gonna like until you've ridden a few. Still, if you're coming from a straight-bar bike, like a hardtail MTB or a hybrid, I'd start here. But, you know what? The more curves and height changes they have, (in other words, the more they resemble drop bars,) the more I like them. The only reason I built my GT Karakoram with generic aluminum North Roads is that I'm broke, and a drop bar conversion will be a couple of hundred bucks. I like it, but drops would be better.

Are drop bars more aero? Yeah, but that's not anywhere near the top of the list of reasons why I like them. I like them because they are the most comfortable. I like them because they work in every situation in which I ride a bicycle, and in most of those situations, they work better than any of the alternatives.

Why drop bars?

Because they're better.

--Shannon