In the beginning, Scrypt was designed to be CPU-only or CPU-mostly. When it comes to cryptocurrency, it promised to be resistant to ASIC and therefore to stay distributed and minable by everyone. Because scrypt is so much more complex, it took them a while, but in the end ASICs emerged and wiped the floor with everyone else.
Nowadays, SHA and Scrypt are basically the same thing as far as you are concerned: you cannot mine them effectively with your computer.