In order to prevent tyranny and monarchy in America, the constitution provided a set of checks and balances to ensure that one branch of the government could not rule by itself. A person serving as both a representative in the house and a member of the supreme court would be working for two of the three branches of the government (legislative and judicial). If one person were allowed to serve for both, they might influence others in both branches to side with them or to further their own political agenda and with support of two of the branches, such a person might become a threat to the system.
