Incumbents spent more than twice as much as challengers. The more opponents threaten incumbents, the more incumbents spend to defend themselves. All of the above. Those running in the most closely contested campaigns for local offices. The best-funded candidates running for national offices.
Candidates concentrate on the states in which they have the least support.
Even if the candidate loses the state, his votes still count toward his national total. To force her opponent to use up time and resources defending that state. To make himself look good to voters in other states. They are likely to help already dominant incumbents at the expense of challengers. All of the above.