why aren't ribbon tweeters more popular?

What is the downside to ribbon tweeters? I haven't had any in a long time (Phase Linear P580's, stolen, still miss them.) Ribbons offer great extension and speed; other than expense, what objectionable qualities limit their use?
Old Apogee Duettas or Divas kill almost anything made today. When rebuilt by Rich Murry at Truesoundworks they are amazing and among the best I have enjoyed. I have listened extensively at friend's system to some of the most expensive finest speakers made today or have tried in these speakers in my own system and keep going back to Apogees. Apogees in stock and or with moded crossovers they kill just about everything. Apogees when re-ribboned they kill everything period. With planar and ribbon speakers you are at a performance with dynamics speakers in most cases you are in a recording studio. Nothing wrong with either venue but that is the difference in sound.
Hi JohnK,

Have you heard/used a ribbon whose sound gets close to the t500mkII?
I actually think the main reason is that they are difficult to integrate with dynamic drivers.

Tonian does it extremely well, somehow.

They also don't have the dynamic snap of a cone tweeter.
The amk2 Is one of the better hi frequency transducers available but RAAL ribbons are wonderful in there own right and 1 can design fine sounding loudspeaker with either transducer. Where design would be best if a amk2 is used a ribbon wouldn't be the best choice, same where a RAAL would work best that design wouldn't be so good for amk2. Hope this makes some sense ;) They are not inter changeable.