Because all states have chosen to allow the electors to be appointed, so to speak, by the voters and the day for the election is uniform throughout the country. If a state decided to have the electors appointed by the legislature, as South Carolina did up until 1860 for example, then the legislature could appoint the electors on any date they decided so long as the electors were chosen in time to cast their voted on the date specified for doing so.
Election Day is one day, but there’s really no such thing as Election Day in many states. Between early voting and voting by mail, “Election Day” is now a weeks-long affair. It’s only the certification of electors in December that must take place on the same day across the entire nation.