A few reasons...
1. After a snake has eaten, it is less able to move about and defend itself for a period of time (depends on how big the meal was) and therefore a natural reaction to stress is to regurge the meal so the snake can move more freely to escape the stressful situation.
2. The digestion process for a snake takes a tremendous amount of energy and it is best for them to let them "sleep it off" as they are naturally inclined to do.
3. The teeth and nails of the prey are among the last things to be digested, and the most dangerous. Handling a full snake may cause punctures along the digestive track. While this is certainly not a common occurrence, it IS a risk that I personally see no need to inflict on my animals.
For me...it's about meeting the animals' needs, rather than my own desires to hold and handle. They prefer to sleep deeply after eating a decent sized meal, and that is what their bodies are created to do. I'd rather minimize risks to health and let them be after eating.