scrypt and sha-256 are different algorithms and in short words: scrypt needs more resources (more and fast RAM) than currently available FPGAs or ASICs don't have.

You can read more here: http://en.wikipedia.org/wiki/Scrypt#Introduction also some more info here: http://security.stackexchange.com/questions/16354/whats-the-advantage-of-using-pbkdf2-vs-sha256-to-generate-an-aes-encryption-key