Why do planars have poor bass extension?

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.

except that I would not describe good cone bass necessarily as "boom-boom-boom"

Mapman, I suppose your key phrase here is "good cone bass"! How many such speaker systems exist today? Most of the speakers in the market are indeed boom-boom-boom. In relative terms only a few do bass well/correctly & many of them are from yester years....

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...

you mean it's easier to match an amplifier to a cone driver speaker than a planar speaker?