you want to offer a wheel that will require minimal effort for ST owners to fit, otherwise you reduce your market and cost of entry.
offsets below 40 are will be tough to fit without further modification especially with proper size tires. the weight of 19" is hard to keep low compared to 18s. also reducing the sidewall height too much doesn't work well for 19s. going lower than 18 would benefit non-ST owners while keeping weight low, but 18s could also work on non-STs. anything wider than 8" is a nice and aggressive option.