re: Why do most bands start off strong and then fade?

The first album could have been written over a span of five or so years, songs perfected while played at small gigs, etc.

The band gets signed, hits the road to promote their album, and is forced to come up with a follow up, on the spot while on the road. Unless you have a group of incredible song writers with an extensive catalog of riffs and ideas, with inspiration coming from all directions, it's hard to come up with something that quick.