Pluto is cold because it is, on average, 3.6 billion miles away from the sun. The sun provides heat and energy to the entire solar system, and the farther away planets are from the sun, the colder they are.
Pluto was discovered in 1930 and was classified as a dwarf planet due to its small size. Just like the other planets that orbit that sun, Pluto has seasons based on its distance from the sun. Pluto orbits the sun in an oval pattern and has average temperatures between -387 degrees Fahrenheit and -369 degrees Fahrenheit based on its position in orbit with the sun.