Why so few speakers with Passive Radiators?


Folks,

What are your thoughts on Passive Radiators in speaker design?

I've had many different speakers (and like many here, have heard countless varieties outside my home), from ported, to sealed, to passive radiator, to transmission line.

In my experience by far the best bass has come from the Thiels I've owned - CS6, 3.7, 2.7 which use passive radiators.  The bass in these designs are punchy yet as tonally controlled, or more, than any other speaker design I've heard.  So I figure the choice of a passive radiator must be involved somehow, and it makes me wonder why more speaker designers don't use this method.  It seems to give some of both worlds: extended bass, no port noise, tonally correct.

And yet, it seems a relatively rare design choice for speaker manufacturers.

Thoughts?
prof
In the technical section of his Rythmik Audio website, designer Peter Ding makes his case against passive radiators in subwoofers.
In the late 1970s (1977) I had a pair of Koss passive radiator loudspeakers.  The radiator (thick plastic passive cone) had a weight one could add to further dampen the radiator.  I drove them with a Dynaco St-150.  They had the fastest bass attack/transient response I have ever heard.  Traded them for a dining room set and purchased Genesis (old east coast sound) passive radiator floor standing speakers.  I moved on to sub/satellite systems but I fondly recall the Koss.  The cabinets were real oak and the tweeter was a plastic thing in a rectangle faceplate.  EV and Polk also made passive radiator designs in the 1970s-1990s.  They take up a lot of vertical space (unless the radiator is mounted in the back).  
A passive radiator speaker may sound slow with a tube amplifier that has a low damping factor. Sealed speakers tightens up the bass by default. 

But, with an amplifier having a good damping factor? A passive radiated speaker can sound as tight as sealed,  but having more dynamics than a sealed can allow for at higher volumes.