America is new. Even in the 90s boulder was largely empty, competition for land was low, so land was cheap. As people spread to newer cities and gained wealth they bid up the price on land.
Uh, no it wasn't? I was living there and continued living there for the next 30 years. It always felt about as dense to me as it did back then.
This is why its so expensive. Demand for housing has increased but supply has not. The government refusing to allow densification in the face of increased demand means prices skyrocket