Mostly because the Sith have goals that in the long run interest the Mandalorians.
They pay well, their enemies are usually powerful, and their wars are usually just what the Mandalorians need gain great stories of battle and valor. Seeing as the Jedi are more of a Defense and Prevention group, they are the exact opposite and why your rarely if ever see Mandalorians working with them.
In fact I think The Clone Wars is about the ONLY time you see the Mandalorians in the employ of the Jedi, where they were training sergeants for the Clone Army and mercenaries for the Republic.
There was also the time during the New Sith Wars. And at some point in time the Mandalorians helped the New Republic During the Yuuzhon Vong War.