Why do snakes only eat once a week?

The simple answer is - energy conservation !

Snakes are cold-blooded - they don't regulate their own temperature like mamals do. In their natural habitat, they would spend a lot of time in the mornings basking on a rock in the sunshine so their muscles are warmed enough for them to hunt. Lots of snakes (especially venomous species) don't go hunting for food, lying in wait to bite their food as it runs past them.

When a snake eats, most of the food is turned into fat which it uses to survive on between meals. Additionally, in the wild they'd have to actively hunt for their food - instead of having it 'dangled' in front of them in captivity. They could go for months without catching anything - so their bodies are adapted to surviving long periods between meals.