Because nature didn't design them that way. Snakes aren't meant to eat something that has been cooked - partially, fully, or any other way other than raw and whole.

Remember that snakes aren't like dogs or cats. We haven't taken them and developed them in certain ways to do certain jobs and be domesticated. Snakes are exactly the same in captivity as they are in the wild - no matter what kind of crazy morph they are, physically they are still the same in the wild. They would never eat cooked food in the wild so nature made them specially designed to swallow and digest their prey whole and freshly raw.