Why is 2 Channel better than multi-channel?

The quality of imaging available in multichannel systems using height information (for example David Chesky's 6.0 audio layout) substantially surpasses what even the best of 2 or 3 channel stereo can do. Bomarc is right in stating that 5.1 is theater-driver and is not optimized for audio.
It will be some time before the recording community and audio industry at large learn how to record, produce and market multichannel audio in a practical way. This is a large subject and cannot be covered here.
However, as multichannel evolves, it's to be hoped that audiophiles will be the first to recognize the potential, since we care more about sound quality than does any other group.

In response to Sean's post, who is the 'they', as in 'they've been trying to get two channel reproduction dialed in for years'?
There are poor recording engineers and companies around, and there are pressing plants that contribute to serious degradation in output, but there is also a strong and fanatically devoted group of recording and mastering engineers who take their jobs with the utmost seriousness, as well as artistry.
Regarding techniques for multichannel audio, there are people currently working with ambisonics, ambiophonics, 7.1 and other non-5.1 multichannel miking approaches, as well as JJ's. Those working in these techniques are in academics and recording studios, as well as the pro audio engineering industry.
There is much to be learned in multichannel. But it is a serious mistake to think noone is working in it, and that noone thinks beyond profitability. The more challenging issues are how to produce and market audio-centric multichannel, and what speaker arrangements (as well as types) make sense.
I like Sean's earlier post in which he suggests that the 'commoner' may be the early adopter. True. Audiophiles may be so heavily vested in two channel, that a younger and more flexible, home theater-acclimated group becomes tomorrow's multichannel audiophiles.