Because all the assassination fists have a core secret that is absent in regular Secret Martial attacks. For example, the vibration in Ten Thousand Qi Evaporation Fist, the splitting in Hundred-Split Fist, and the water breaking in Crack Water Fist. Their creation is based on this core secret, which is entirely different from the theme of regular Secret Martial attacks.
Everything about the assassination fists is to get closer to the core secret, to better use the core secret. Just like Bai Xiao's Ten Thousand Qi Evaporation, the so-called Water Bird Soaring is a combination of airflow and vibration. Whereas Heavenly Bird Destruction is a combination of momentum and vibration, and Extreme Bird Burning Sky is a combination of flame and vibration.
Therefore, Battle Divine Power cannot take effect against assassination fists.
