It is increasingly trivial to try millions or billions of hashes per second. Using scrypt forces the computer to use more resources (in this case, memory in particular) which means that scaling the guess rate is orders of magnitude slower and cannot be significantly increased by building faster computers (because it's limited by memory not process speed).

http://en.wikipedia.org/wiki/Scrypt

It comes down to this: Why use something which is designed for speed like hashes to do something that can be broken by highspeed guessing?