The founders wanted the winner to have wide support across the country, rather than deep support in just a few states, as a way of avoiding civil war.
In the election of 1860, the anti-Lincoln vote was split among three candidates, resulting in Lincoln winning with just 39.8% of the popular vote, and all his support in the North. The Civil War followed immediately.
Maybe so, but then maybe not. The Constitution seems to have been written with the expectation that the Electoral College would not be sufficient to elect the president on a regular basis. In early decades of the U.S. it was not uncommon for three or more major party candidates to compete in a presidential election, so the potential for plurality results was much higher back then.