Why are there so few Force sensitive Mandalorians?

• Mandalorians are not a species but it is more of a philosophy. Anybody having this philosophy can join the Mandalorians.
• Mandalorians are not part of the Republic, they've even fought quite often against the Jedi. Jedi can therefore not test Mandalorians for Force-sensitivity. People may therefore be Force-sensitive without knowing it.

From the Wookieepedia article on Mandalorians:

Mandalorians were a predominantly human ethnolinguistic cultural group who originated on the planet Mandalore. Mandalorians had a particularly unique role in galactic history as legendary warriors against the Jedi, and could be commonly found not only on the Mandalorian homeworld and its moon, Concordia, but across Mandalorian Space and the galaxy at large on worlds such as Kalevala, Krownest, and Concord Dawn. Mandalore had a largely martial history, but by the time of the Clone Wars the pacifist and reformist New Mandalorian political faction controlled Mandalore's government, led by Duchess Satine Kryze. This led to internal conflict with other Mandalorian groups like Death Watch, who wanted to maintain the warrior ways of their Mandalorian heritage.

...

Their first confrontations with the Jedi Knights took the Mandalorians by surprise: their use of the Force and the powers it granted them was a challenge that they had never seen before. However, the prideful nature of the Mandalorians would not allow that disadvantage to stand, and the technologically advanced people began to devise ways to overcome the powers of the enigmatic Jedi. The result was a major advancement in the development of Mandalorian technology, specifically of their arsenal and their infamous armor. They also developed a fighting style that combined their entire arsenal and skills to allow them to counteract the Jedi's supernatural abilities. With these new technologies, the Mandalorians began to win their share of conflicts with the Jedi, and their warriors earned the reputation of being capable of confronting and defeating Jedi Knights.

It is not that Mandalorians are less Force-sensitive than other races but they are not tested. It is the same with the Outer Rim. You won't find a lot of Jedi being from there as they are not part of the Republic. In The Phantom Menace, they mention that Anakin would've been found earlier if he was born in the Republic. It's the same with Mandalorians.