Spoiler: To prevent Trump from pardoning himself, his family, his friends, and his associates.
nytimes.com/interactive/20…
Double Jeopardy is from the Fifth Amendment & prohibits a person from being prosecuted twice for the same crime. law.cornell.edu/wex/double_jeo…
This may explain why Mueller is charging very few crimes arising from narrow facts, even though it's obvious...
Side note on pardons: It is highly unlikely that Trump would get away with issuing pardons for himself or co-conspirators.
Amending the double jeopardy law is a neater solution, avoiding time-consuming court battle.