why aren't ribbon tweeters more popular?

A lot of speakers have ribbon tweeters these days, it not really uncommon at all. Like any other technology, some people like them and some don't.

Downsides—delicate, easily overdriven, higher distortion, beaming dispersion pattern. Ribbon and quasi-ribbon also exhibit edge resonances and a rising frequency response, some more than others, making them difficult to work with and listen to. However, these traits are attractive to aging boomer ears with hearing loss.

Ojgalli Only thing I agree with is delicate but they can be easily repaired unlike other transducers. Ribbons can image wonderfully can produce accurate sound and measure well. Sure are not difficult to work with I do so daily. But some have such biases they can not except the fact that ribbons are just like any other transducer used in audio and if done right a hi quality loudspeaker can be designed with ribbons or most any type of transducer. Happy listening.

Evolution Acoustics and Aerial Acoustics use these on their top flagship speakers.
Nuff said!