[trinhn812]

I don't have any clue why?

[Yggdrasil]

Well, consider the difference in structure.  Water has two hydrogens attached to an oxygen atom arranged in such a way that the dipoles of the O-H bond add constructively.  In isopropyl alcohol, one of the H atoms is replaced by a hydrocarbon group.  Hydrocarbons are non polar, so the presence of the hydrocarbon group makes isopropyl alcohol less polar.  Furthermore, only one O-H dipole contributes to the overall dipole of the molecule as opposed to two O-H dipoles in water.