Quite possibly because of :
a. emissions regulations (but Suzuki markets the S40 in the US, while they binned it from the EU lineup)
b. they didin't sell in large enough quantities to make it a convenient import
c. all of the above.
I, for one, would like to see an 800cc "S50", at least as a "custom-to-order" upgrade for the S40.
Possibly with a rear disc brake and 6-speed gearbiox, by the way...