why is the direction of dipole movement from negative charge to positive charge?

The torque on a dipole is given by,

The torque obtained is positive when the direction of the dipole is taken from negative charge to positive charge.

Another reason is, in analogy with magnetic dipole where field outside the magnet is from North to South while field within the magnet is from South to North, the direction of electric dipole within the dipole is taken from negative charge to positive charge (this is just an analogy that is followed, in practical direction of electric lines of force is always from positive to negative charge).