Q:

Why does Neptune only have one major moon?

Neptune is profoundly anomalous when it comes to moons among the gas giants.  Jupiter has its four massive moons, Saturn has a several large spherical bodies and Uranus, a world similar in size to Neptune has five or so major moons.
Yet Neptune, the ice giant which sports one of the largest hill sphere radii in the solar system only has one major moon, of which is likely to be ripped to shreds in the future.
I find it peculiar because Uranus has five major moons yet Neptune only has one major moon.  What explains this disparity?  Did some disruption happen to earlier moons Neptune may have had?  What's known about this?

A:

It seems possible that Neptune once had a system of moons similar to other outer planets. However Triton, a Kuiper belt object was captured by Neptune, and this disrupted the existing moons. It would have initially had an eccentric orbit, intersecting with the orbits of other moons, setting them into unstable orbits and ultimately either colliding with each other, hitting the planet or being ejected. 
The minor moons formed from the debris of these collisions after Triton had entered a more stable orbit. So the disruption was the capture of Triton.

