What causes Mars and Saturn to have years of different lengths?

The cause is the fact that they have orbits of different size.

For two objects in orbit around the same central body, the one in the

larger orbit will take longer for each complete revolution.

And if two objects are in the same-size orbit, their periods of revolution

are identical, even if the objects have different size and mass.

(Example: All TV satellites are in the same orbit. Some are big and heavy,

some are smaller and lighter, but all of them revolve around the Earth in

exactly 24 hours.)

Because it takes longer for Saturn to revolve around the sun because it is positioned further away from the sun