How come Horn + woofer designs are not more popular?

Good speakers play everything well and make bad recordings sound better even if they are not as good as the best recordings. There are good examples of every type of speaker (except the Hill Plasmatronic.) Mostly what you like depends on what you were exposed too. If you have only heard crappy horn speakers that is the opinion you will have, same for ESLs. Most people have dynamic speakers because they are easier to build, can be more aesthetically pleasing and are generally smaller than the best examples of horns or ESLs. My problem with most speakers is that they are hard to integrate into a normal size room acoustically. The room and speaker are a system. Some speakers will never sound good and some rooms will never sound good. Thinking about my self I go with what I have learned to work with and know I can get to work to my expectation in my own room. Making mistakes gets far too expensive at this level and I have already made enough of them.

Oh, back on topic, there is a compromise in a two way horn system that is difficult to get around. You either have to run a woofer up into the midrange or make a very large horn to get down to where most woofers do well. I do not think there is a satisfactory middle ground. Look how large a K Horn midrange horn has to be just to get down to 500 Hz were it crosses to a 15 inch woofer. This is one area were that ESLs have a major advantage over both horns and dynamic speakers. You can easily design a speaker that is One Way all the way down to 100 Hz as long as you can live with the size. 

Two words: too big.

If not for tube amplifiers there would probably be little need for this typically high efficiency design in most homes. High efficiency potential is the key advantage. Pro audio involving larger spaces is a different story.

Horns - They're not more popular because they sound terrible, the reason is that simple.