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
@rja2-  Refer to my post, dated 1-30, for the answer.
Post removed 
Jim T and Richard V seperated at birth my theory, sold and own both. RIP Jim and Dick Hardesty, who was absolutely correct in the time period he wrote it. The Treo is a poor mans Quattro and has a downward firing port. The have impressively tight and refined bass for a relatively small speaker. @tommcarr the acoustic couplers on the 2 and 3 are active drivers with a narrow passband, designed to couple to room via rear wall. Provided the amp has some grunt and control they IMO provide great bass extension.

aint nothing free, the port and a passive both incur phase and impulse penalty at the benefit of extension.

one of my fave old skool passives was the KEF 104ab