Another part, as was mentioned above, is that more time end money is invested into games. Programmers have the luxury of testing teams, and have the entire day to work on a game, not just a few hours after school. The final reason is that the Game Boy is designed to play games. Everytime you put a new cartridge in your Game Boy, you are complete replacing the software. You don't have to worry about a calculator OS getting corrupted, other progams trashing memory locations, flags, etc.
All code is contained entirely in the cart.
There are no ROM calls, libraries, etc., to go wrong. It's all your code running. Wow, this post is long. I hope it clears some stuff for people.