Why so few speakers with Passive Radiators?

I consider the passive radiator a special case of a vented design:

1 Sealed enclosures
2 Vented enclosures
    2a Ported
    2b Passive radiator

Keep in mind I'm just a bozo typing opinions on a forum, not an expert!

Hey man…all bozos are welcome on THIS bus…Well designed passive radiators do fall into the "venting" category although you'd think they imply a sealed box, but the passive radiator is sort of the "unsealed" part part of the thing. I own a discontinued Mackie HR120 sub that was designed for home use (not the "drag it around to gigs" P.A. stuff), and it has a 12" passive on it, unlike the pro stuff I use in some live sound situations that have a vented/ported design (and generally 1000 watts into an 18" driver)…both work well, but the HR120 with a 500 watt A/B amp driving a very beefy downloaded EAW 12" (92 lbs…you just don't want to move it) is flat to 19HZ. REL does well with their passive radiating little subs (my 2 RELs don't have those, but work anyway), as do many others of course, and it's simply another flavor…personally I prefer raw drivers suspended from fishing wire 2 feet from my head, but that's just me. A bozo.

Wait just a minute, you may indeed be a bozo but not as big a bozo as me.  I've only been at this umpteen years and I don't know vented from sealed.....I occupy the front seat on the bus!

I suspect the reason is mostly that ports are cheaper/easier.  I only know of the below three factors.

passive radiator
 - no port noise
 - less sound escapes through the diaphragm than would through a port

port
 - higher efficiency

@jon_5912 that sounds right. From Richard Hardesty in APJ Vol. 3:

Vented subwoofers offer inferior performance compared to sealed enclosure subwoofers of equivalent quality in the following areas: transient response, phase response, group delay,and low frequency extension. You might ask why anyone would choose to make a vented subwoofer. There is one very good reason—high output. The one advantage that vented enclosure designs offer is a reduction in cone excursion at low frequencies.Reduced cone excursion allows vented designs to play louder. A side benefit is that reduced excursion will also produce lower steady-state harmonic distortion measurements.

and Hardesty in his review of the Thiel CS6 in APJ Vol. 8:

They have vented bass loading utilizing passive radiators rather than ports. Alignment is unusual and bass is tightly controlled with little evidence that the enclosures are not sealed.

So, Hardesty was a big fan of sealed enclosures (and even bigger fan of Vandersteens) yet he implied that the Thiel's passive radiator sounded not too different from that of a sealed enclosure. In turn, this implies that Thiel's passive radiator is a "good" trade-off between sealed and ported enclosures. At least that's *this* bozo's take  :^)