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

Focal is of the opinion bi-amping defeats the voicing of their crossovers. Frankly, I think it's a cheap gimmick. You see it all the time on mediocre speakers like Polk RTi and ilk of that range as if someone is buying $1000 speakers and putting $2000 worth of amplifier behind them. Not something I'd ever consider. I highly doubt I can do better than the engineers that voiced my speakers. 

There are reasons to bi-amp. The chief reason is that crossovers are reactive components and they reduce the control and response the amp can have over the drivers. On the amp side, multiple amps can provide better separation because all the power supplies are discreet to each speaker and/or range. Some speakers are designed to be run like that, others not. A well designed crossover can ameliorate various failings and more precisely control voicing. They're not cheap either. 
It's a personal choice. If you don't get hard micromanaging every facet of your system, and I don't, the world has great single amp options that make lots of people very happy. 

The intent isn't so much to use different types of amps for different sections. The intent is to drive different speakers or sections on isolated power supplies. The less interaction the channels have, the better their separation. That's why some amps use separate transformers and power supplies for each channel. I might try that at some point when I get my F5 operational just to see if there's a difference. 

@shadorne 
What!?!?  There's nothing mechanically or electrically wrong with RCA, and the virtue of XLR isn't found in the intrinsic quality of it's cable or connectors. It's found in the virtue of of how 2 opposing signals can be used to cancel noise and distortion and XLR is just a more convenient that pairs of RCA, but pairs or RCA can do the exact same thing.