Question

Why does the electoral college not allow citizens living in U.S. territories to vote, as it is the only place in the universe (astronauts have voted) that U.S. citizens can't vote.

Answer 1

Because the electoral college is limited to a certain number by the U.S. Constitution, it would take a Constitutional Amendment to change this number. Apparently there has not been enough interest in letting the people in the territories be represented by the Electoral College to propose and pass an Amendment.

http://www.archives.gov/federal-register/electoral-college/about.html

http://www.archives.gov/federal-register/electoral-college/faq.html#538

Answer 2

with the exception of Washington DC Only states get electoral vote equaling the number of Representative in congress. Tn has 9 Reps and two senators so 11 electors is given to Tn. Astronauts, military or any one else can vote absentee being citizens of a state.