Because according to state laws, the party that wins the popular statewide vote gets to seat their electors in the Electoral college. Now it has happened where both parties attempted to seat their electors in the electoral college and states returned conflicting results. That led to the Compromise of 1877 (google it, interesting story).
Must electors vote for the candidate who won their State’s popular vote?
There is no Constitutional provision or Federal law that requires electors to vote according to the results of the popular vote in their States. Some States, however, require electors to cast their votes according to the popular vote. These pledges fall into two categories—electors bound by State law and those bound by pledges to political parties.
Which States bind electors to popular vote results? Refer to Electors Bound by State Law and Pledges to find out.
The Supreme Court has held that the Constitution does not require that electors be completely free to act as they choose and therefore, political parties may extract pledges from electors to vote for the parties’ nominees. Some State laws provide that so-called “faithless electors” may be subject to fines or may be disqualified for casting an invalid vote and be replaced by a substitute elector. The Supreme Court has not specifically ruled on the question of whether pledges and penalties for failure to vote as pledged may be enforced under the Constitution. No elector has ever been prosecuted for failing to vote as pledged.
Today, it is rare for electors to disregard the popular vote by casting their electoral vote for someone other than their party’s candidate. Electors generally hold a leadership position in their party or were chosen to recognize years of loyal service to the party. Throughout our history as a nation, more than 99 percent of electors have voted as pledged.
Well the legalese question is, is illegal to [i]run[/i] or is it illegal to [i]serve[/i]?