Why Design New Processor Cores?

Gus asks: "Why do vendors continue to develop new processor cores, as opposed to using newer technologies in older proven designs? The best example of this I can find is IBM selling the 'Blue Lightening' chip, which was really a 386, but was sold as a 486. Why not make a 386 today using copper and a 0.18 micron process? Wouldn't this be fairly cheap? Are new instructions worth adding?"