Hoppiness is partly a reaction to the variance in hops and a pride in "American beer". Cascade hops in particular are cheap, available, and have a strong and distinctive flavor; hence the popularity of IPAs on the west coast, and the presence of companies like Sierra Nevada in the hoppy vanguard.

Even as the Cascade trend has diminished, lots of people have 'reacted' by rediscovering other hop flavors and advertising things like Nobel hopped beers.

Having said that, it's all getting a bit played out (except on the west coast, perhaps) and I'm starting to see more experimentation with other stylings like wheat, fruit tones, and dark ales.