A very stiff tire at lower pressures will still feel slow, whereas a really nice flexible tire at lower pressures will give a much nicer ride, will be faster over rough surfaces (cuz of the suspension effect not removing energy from the frame going over bumps) and will also roll more efficiently on flat pavement--but yup, you got it, they tend to cost more.
I think people miss that point (or maybe just don't care about ride feel). A nice compliant sidewall makes a ton of difference and is one of the biggest factors in how pleasant a bike is to ride IMO. Weight is a big deal as well, but I am not sure a supple sidewall isn't the more noticeable difference when riding.