I think the two go together don't they? I am sure that if the demand was there it would be catered for by the big brands and the retailers. However it is interesting that there are more and more bikes like the AfricaBike available, marketed as utility bikes and not sex-specific. For hybrids, maybe the demand is catered for by there being very little difference - a few cm. maybe - between some of the steeply sloped compact hybrid frames and their female-specific version. For a more expensive bike I am sure a step through frame makes it less easy to sell second hand, though 'pre-owned' WS road-bikes are always in demand.
In fact I get the impression that there are more and more womens' road-bikes available, but that is more a question of size and proportion - correct fit - not step through frames.