C-H,
The primary reason is that 3 phase equipment is typically better quality (re: more rugged, built with higher AIC raitings, etc) than residential grade eqpt. Though in recent years, our commercial/industrial stuff has been arriving from Mexico, Puerto Rico, etc and the quality has suffered.

A lot of residential stuff is now sold though mass merchandisers like Home Depot and Lowes, which have pushed the prices (and quality) down even further.

A few examples:
Square D "Homeline" 20 amp SP breaker $3.50
Square D 277 volt 20 amp SP breaker $57.00

These are wholesale prices off the top of my head.

I can dig up my catalogs to find out more if you like.

Paul,
Or I can just convince the poco to run a few more high voltage legs and transformers down my street! [Linked Image] I'm sure they will.... [Linked Image]

Peter