Because then whichever party currently has the support of the majority of the country would win every seat. Yes, you could do things like stagger elections, but overall I think it'd be worse: say each representative has a six-year term and there are six annual elections, you basically have six blocs in Congress instead of 435 members.
And of course the United States comprise a federal republic; most issues simply shouldn't be federal, and the vast majority are definitely not national. Who outside of Massachusetts cares if Massachusetts makes wearing a leek on St. David's Day compulsory or forbidden?