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

bi-amping can be great but it depends on the application.   there really are not many disadvantages for speaker makers except the slight expense to add the additional wiring, binding posts and jumpers.  
the advantages are the ability to have a more dynamic presentation through increased amplifier power, the ability to isolate the tweeters and midrange drivers from the woofer amplifier which may clip at higher volumes.  the clipping of the woofer amp basically becomes inaudible because it is subject to the woofer's crossover.  The midrange / tweeter amp will never clip because of the reduced demand on the amplifier to play those frequencies.  
the ampfliers need to have the same gain or the ability to be level matched.  doing so will not affect the voicing of the crossovers.  
i had an incredibly dynamic powerful sounding system with PSB tower speakers seeing 150 watts on the woofers and 100 watts at the tweeter / midrange, amplifier gain was identical.  the system was able to play much louder and had no audible sense of strain, compression or distortion.  

I had my first bi-amping epiphany today. I never believed it is useful to do with just two dumb power amps. I was bored today and connected two integrated amps with tone controls to my speakers. I let Ayre run my treble and then vintage Marantz to run my mids and bass. I adjusted tone controls for more bass, adjusted balance since LP was playing uneven in volume. I was shocked how much more decent bass I have extracted from my speakers that were always light in that department. The difference is HUGE. So if you want to play around with bi-amping try two integrated amps with at least VOLUME control, so that you can adjust the sound for the type of music you are playing. My speakers came alive after 8 years for the first time. I listened to some classic jazz and  Leonard Cohen, it sounded so, so good.... I am going to do the same to Wilsons when my Luxman returns from recap service.

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.