Why do planars have poor bass extension?

The 20.7 (and the prior 20.1) has a magnet structure on both front and back of the diaphram to improve bass performance and slam. (lesser models have the structure on only one surface)
The main reason for lesser bass performance is the fact it is a dipole. The bass from the rear meets the bass from the front at the sides and self cancels a lot of the bass.
Any full dipole is gong to have the same issue. That is why you see so many with a cone woofer stuck on a dipole.

One trick is to add 'wings' to the sides and the bass gets better, as it has more distance to go to self cancel at the sides.
I use a 6" panel addition on my Magnepan 3.6s just off the bass side (none on the tweeter side)

Also the panels flex, and the various stiffening stands also improve the bass response.

I've read accounts that make sense to me that often the support structure is the weak link in the bass of planars. The frames tend to not be very massive and there is little inertial foundation for the drivers to work against. Adding mass and/or rigidity to the frame helps. At least that's what I've read that seems to make sense.

Also, many do not realize that most planars like Magnepan forexample require lots of power (though not necessarily current) to be driven to the max. A 40-80 watt SS amp might sound OK, but does not hit the mark. 200W+ SS or a larger tube amp is usually what is needed to wake things up.

Not all planers have poor bass. My Apogees have plenty of bass, but it is different bass than a cone speaker. It is tight and defined and tuneful and goes deep, but it just doesn't have that hit you in the chest slam of a good cone woofer.

Why do planars have poor bass extension?
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Sideshowgabe

maybe because you've never heard a good planar setup?? I'm with Ptmconsulting on this matter - my bass is fast, tight, tuneful & goes as low as the room will support (about 40Hz) but it does not sound like a cone woofer boom-boom-boom bass.
2nd reason you could be disillusioned with planar bass is that planar bass sounds different from cone boom-boom-boom bass which is the only thing that 99% of the people know. So, when you hear planar bass, you ask 'where's the bass?' It's all in there just that it does not call attention to itself like most cone woofers do.

I'm in agreement with PTM and Bombay, except that I would not describe good cone bass necessarily as "boom-boom-boom". I realized after owning planars for a while that "cone" bass can sound similar to planars when done right, for example with good monitors set up and driven properly.

However it may be easier in general to coax better dynamics out along with good bass articulation ala planars using many common amplifiers out there with cone bass done well than with planars, if that is something that matters to you. I find it does, especially for more rock/pop types of music and large scale orchestral works and big band music, for example. I settled on the OHMs largely for their planar or electrostatic like sonic attributes combined with cone bass type dynamics (albeit with a different, more omni-like spatial presentation).