Please explain why are we not supposed to swallow toothpaste?

It's okay. It's okay to swallow toothpaste. We just tell children not to swallow it so they don't get too much Fluoride that can cause some minor discolorations in the enamel.
Ingredients. Toothpaste is not formulated to be ingested. It is designed to be used in the mouth, and then spit out. Ingestion of those ingredients can upset your stomach.