Why in normal stationary phase the principle of separation is mainly adsorption while in reversed phase it's partition ?

In case of Normal Phase. In a  non-polar environment, hydrophilic molecules will tend to associate with each other (like water drops on an oily surface). The hydrophilic molecules in the mobile phase will tend to adsorb to the surface on the inside and outside of a particle if that surface is also hydrophilic. Increasing the polarity of the mobile phase will subsequently decrease the adsorption and ultimately cause the sample molecules to exit the column. This mechanism is called Normal Phase Chromatography. It is a very powerful technique that often requires non-polar solvents. Due to safety and environmental concerns this mode is used mostly as an analytical technique and not for process applications.

Reversed Phase

The opposite of normal phase, or Reversed Phase Chromatography, results from the adsorption of hydrophobic molecules onto a hydrophobic solid support in a polar mobile phase. Decreasing the mobile phase polarity by adding more organic solvent reduces the hydrophobic interaction between the solute and the solid support resulting in de-sorption. The more hydrophobic the molecule the more time it will spend on the solid support and the higher the concentration of organic solvent that is required to promote de-sorption.