Why can't Pluto collide with Neptune?

Pluto and Neptune can never collide because their orbits have a significant incline with respect to one another. They also have a gravitational resonance that causes them to alter speed as they near one another. As a result, they never get closer than 2.6 billion kilometers.

Two-dimensional representations of the orbits of Pluto and Neptune show intersection points. However, in three-dimensional space they regularly switch places, with respect to distance from the Sun, without ever getting near each other. The gravitational resonance between them comes from Neptune orbiting the Sun much more quickly than Pluto, making three trips around the sun for every two that Pluto completes.