Why so few speakers with Passive Radiators?

The 2Ce and 3A use sealed enclosures (I think the Treo has a downward firing port). From Tom Norton's 3A review in stereophile:

A cutaway photo reveals four drivers and three separate sub-enclosures—the bottom of which is the largest, for the rear-facing 10" driver that Vandersteen refers to as an "Active Acoustic Coupler." It is, as the name implies, actively driven [though it shares some of the characteristics of a passive radiator—Ed.]. Since the AAC is designed to cover primarily the range below 35Hz, I would be inclined to call it an integral subwoofer; but perhaps Vandersteen chose to give it another designation to distinguish it from the company's dedicated, outboard subwoofers. 

I think the crossover is designed as a 3-way, so not sure how this thing works - and I ran 2Ce Sig IIs for 10 years! Maybe the filter for the woofer has a sub-network for the AAC?

Hi guys,   Well,  a lot great info, a few minor inaccuracy's... Even the Richard Hardesty post is more opinion and not cast in stone. 
Yes,  A passive radiator is more of a vented or ported design.  When we do a ported speaker,  Lengthening the port lowers the frequency of air moving through the port so changes the frequency the woofer peaks at.  Where Richard was not quite accurate in his description is that it is completely feasible to port/vent a driver to be very smooth and accurate bass, it is then a function of a well damped amplifier to control it. There are many ported speakers out there that are great. A passive radiator works similarly in that as you add mass to the radiator,  it will like a port, change frequency where there is a peak or dip and by how much. Every driver does change spec as it is used over the years and fact is,  a sealed box comes much closer in keeping its original sound as it wears and a ported box is the same.   Oddly enough in my own speakers at home right now, I have 3 pair built. A 10 inch 3 way in a sealed box,  an 7 inch mtm in a ported box and a 12 inch 2 way (big Heil amt) in a passive radiator box.  All 3 are flat, accurate and fast with the right equipment in front of them.  

@beetlemania, thank you!

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).