why was the election of 1800 decided in the house of representatives?

The election of 1800 was decided in the House of Representatives because no candidate received a majority in the Electoral College.

In that election, Thomas Jefferson and Aaron Burr both received the same number of electoral votes (73), leading to a tie. The process for resolving such a tie, as originally outlined in the U.S. Constitution, required the House of Representatives to decide the outcome. This situation ultimately led to the election being resolved after multiple ballots in the House, with Jefferson becoming the President and Burr the Vice President.

It is worth noting that the experience of the election of 1800 contributed to the passage of the 12th Amendment in 1804, which established separate ballots for the President and Vice President to prevent similar situations in the future. However, the immediate reason the election was decided in the House was the tie in the Electoral College.