Why is pluto so different from other outer planets

It is different because it is so much smaller, it is made up of different material than the outer planets, and it does not orbit the sun.

Pluto's Unusual Orbit. It takes 248 Earth years for Pluto to complete one orbit around the Sun. Its orbital path doesn't lie in the same plane as the eight planets, but is inclined at an angle of 17°. Its orbit is also more oval-shaped, or elliptical, than those of the planets.