Reading up on prokaryotes recently, I discovered that organisms can move with just one flagellum but they need several cilia to move. Why? Can organisms move with just one cilium?

This isn't the most technical answer, but cilia are like tiny little hairs that work together like oars to propel the bacterium. A single cilium isn't large or powerful enough to propel on its own. A flagellum is more like a single tail that is capable of propelling on its own (think sperm), but a cell can have more than just one flagellum in different arrangements.

Is it due to the difference in their length?

yes cilia are much much smaller than flagella

To elaborate further, think of cilia like legs on a centipede. Now think of a centipede with one leg: it's going in very slow circles.

As for a flagellum, it virtually is a propellor, like a screw on a large ship.

Cilia are more like oars, and flagella are more like propellors. A single oar will cause movement around in circles. A single propellor will do its job quite nicely.