it is largely a waste of resources to dedicate time and engery into states that are decidedly Republican or Democrat. By focusing on swing states, a candidate can have more of an impact on their election chances by earning electoral college votes in states that are on the fence. It is very unlikely that a Republican Candidate will win a state like California, also, it is very unlikely that a Republican will lose a state like Oklahoma, so they focus on states that can tip either way.
