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?
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.
I think the down sides are dispersion patterns (in some designs) and difficulty integrating them. However when implemented well they I find them excellent.