Because Hokuto Shinken is not only Martial Art & Art of assassination but Arts of all techniques.
Here is my theory:
With Hokuto Shinken your aura nodes will be unlocked and you are complete master of Nen.
With Hokuto Shinken your willpower be strong and you are complete master of Haki.
With Hokuto Shinken your chakra is unlimited and you can achieve all type of chakra nature and you are complete master of ninjutsu, genjutsu, taijutsu.
With Hokuto Shinken your spiritual energy will be strong and you can see Shinigami, Stands, Ghost and complete master of Reiatsu. Because Hokuto Shinken not just make you physically strong but mentally too.
With Hokuto Shinken your life energy is unlimited and you are complete master of Ki.
With Hokuto Shinken you are immune to magic.
With Hokuto Shinken you are complete master of cosmo. Because Hokuto means North Star and this art style based on constellation called Ursa Major and power of cosmo is come from constellation.
But combined with Hamon and Rokushiki it was more deadliest.
Because Humans can only use 30% of their potential power. However the secret of Hokuto Shinken tapping the other 70%!
So this is my theory.