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

Imagine if you did it with two amps of the same make/sound.  That's where the true magic is.  I have heard a few speakers bi and even tri amped with NAD amps.  Just takes some systems to a new level.  As was stated above, no feedback messing with other drivers and each amp is freed up to only worry about a segment of the market if you would. Makes a huge difference, but in the US, most want so much control over their systems, they won't even look at semi or fully active speakers which is too bad.  At least with fully active you hear them the way the designer built them and if you don't like them, move onto other's you do like.

My one experience was positive but perhaps not that common. Many years ago I had a pair of Brian Cheney's VMPS Supertowers, a fairly ambitious 5(?)-way design where the bi-amp option allowed for a 2nd amp to drive the woofers separate from the mids and tweeter array. This worked quite well as it allowed dialing in a better balanced bass in room.

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

Because their designers are not confident in the engineering skills of their customers.

In the manual for my new (to me) Vandersteen 2Ci speakers, their recommended version of (vertical) bi-amping is using two stereo amplifiers each driving one speaker with the left output driving the tweeter/midrange and the right connected to the bass. Maybe this is common but I'd not seen this sort of configuration before.

"Hypocrisy"