Why so few speakers with Passive Radiators?

Thanks, I’m aware that everything is trade offs, but the passive radiator seems such a successful trade off I’m intrigued that it’s not used more, vs the much more common trade off of, say, ported.

Having owned and listened to many sealed designs, I don’t find the passive radiated Thiel bass remotely describable as "slow." It’s completely the opposite.  I've searched in vain for other speakers with as tight, tonally controlled bass as I hear from this passive radiator design.

tomtheil wrote:

• whereas deep bass benefits from near-the-floor coupling.

This is one reason I’ve viewed some of the many boutique speaker platform/feet with a bit of suspicion. Many report sonic change in their speakers upon using these feet, but it seems obvious to me that raising the speaker off the ground is going to change the relationship of the bass with floor coupling and...change the sound. Not to mention raising the mid/tweeters higher in relation to the listener’s ears.  This is changing the, I presume, from the floor relationship the speaker was designed for (placement of woofer height relative to ground, etc).

So it’s not surprising sound will change...but...for the better? Haven’t you changed the relationship the designer created with the floor to something else?

I recently tried some iso-acoustic iso-puck footers under my Thiel speakers and found pretty much those changes - a change in bass and the high end/mids. The bass actually because a bit more "overhang"-like and fluffy, and tightened right back up again when I just put the speakers directly on the floor. (I’m not using spikes).

Though, on the other hand, I’m wondering if speakers are generally voiced with spikes underneath? If so, I suppose as long as new footers don’t alter the height beyond what you’d have with the spikes, it should be a detriment to the sound.