Board Thread:Questions and Answers/@comment-29616136-20160813104312/@comment-27701762-20160813135835

There are a several possible explanations, all of which may be possible simultaneously.

The door at the end of the Ruins is described as a one-way exit, meaning that the door itself can't be used to re-enter the Ruins. There is, presumably, a way to get back into the Ruins, but that way could be difficult or long, making a trip to Snowdin taxing.

Toriel has no reason to go to Snowdin. Everything she needs can be retrieved from the Ruins, and she wants to make sure that she stays in the Ruins to find any human children that fall into Mt. Ebott.

Toriel is also on a self-imposed exile, since she is disgusted with Asgore's plan to kill seven human children and use their souls to destroy the Barrier. Going to Snowdin may signify returning to Asgore's realm, which would be something Toriel wishes to avoid as part of her decision.