If bi-amping is so great, why do some high end speakers not support it?

Adding (a) powered sub(s) with built-in x/o filters IS a form of bi-amping, but doing so does NOT replace the speaker-level x/o, which is left in place to divide the signal for the speakers’ (not the subs’) woofers vs. tweeters.

There has been mention here of digital active x/o’s, but there are still analog x/o’s available, for anyone not wishing to turn his LP’s, 78’s, 45’s, tapes, and FM signal into digits. Two reasonably-priced good ones are made by Marchand and First Watt (Nelson Pass), both around a grand. Bi-amping works only with speakers designed to be so used---Maggie 20.1’s, for example, are, 20.7’s are not (just as 3.6’s are and 3.7’s are not, same with 1.6’s vs. 1.7’s).

The pre-.7 Maggies can be bi-amped using an active electronic x/o because their stock speaker-level x/o's are textbook parallel designs, easily duplicated by an outboard x/o. The .7 speakers have series x/o, not so easily duplicated. Any speaker having corrective filters (Zobel networks, to correct for any driver misbehavior) also make bi-amping complicated.

One reason bi-amping can improve the sound of a speaker that has not been mentioned here (I don't believe), is that powering each driver with a separate, dedicated amp prevents the back-emf (electro-magnetic force) that a woofer sends back to the amp powering it can not reach the tweeter (in a 2-way loudspeaker). The advantage of that arrangement should be obvious!

There are sonic benefits to be gained by getting rid of the speaker-level crossover in a loudspeaker, instead filtering the signal before the power amps, the amps then powering the drivers directly. That requires the speaker be designed to be used in such a fashion, with the line-level crossover duplicating the standard speaker-level one, with no compensation networks for the drivers.

Bi-amping was recommended (by both Magnepan and their original distributor, ARC) for the pre-series .7 Magneplanars. The series .7 Maggies can not be bi-amped, for two reasons: The necessary speaker cable connectors are not present, and, more importantly, the series .7 crossovers are of series design, unlike those used in the pre-series .7 Maggies, which does not allow a simple "textbook" (1st, 2nd, 3rd, or 4th order filters) crossover.