Why do planars have poor bass extension?

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).

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?

"Mapman, I suppose your key phrase here is "good cone bass"! How many such speaker systems exist today? "

I suppose what is considered good is subjective, but I would say many if set up well and driven well with the right amp.

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

I'd say smaller, less powerful, and accordingly less expensive amps in any particular line might be found to fit the bill better in general with many box designs compared to planars. At least that is what I have experienced.

Newer high performance amp technologies like Class D switching amps where you essentially get a bigger high performance amp in a smaller package might help render that distinction as more of a moot point though I suppose.

Good bass extension can be achieved with adequate power.

If the bass area is large enough they can have a lot of bass. My Apogee Duetta Signatures have very impressive bass and the bigger Apogees had even more.