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Why direction of dipole is from negative charge to positive charge?

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The general definition of dipole moment is $$\vec{p}=\int \rho(\vec{r}) \vec{r} dV$$ For the special case of a pair of charges $q$ and $-q$, it becomes $$\vec{p}=q\vec{r}_+ +(-q)\vec{r}_-$$ $$=q(\vec{r}_+ - \vec{r}_-)$$ $$=q\vec{d}$$ where $$\vec{d}=\vec{r}_+ - \vec{r}_-$$ is the vector pointing from $-q$ to $q$.

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It's really just convention. There is nothing more valid in defining the dipole moment to be from negative to positive as from positive to negative, as long as the definition is consistent. (Though, to be honest, from negative to positive makes more sense (the direction an electron would move) than from positive to negative, which is the convention of direction of current flow.)

Point 2. in this question addresses this well: Why does the direction of a dipole moment go from negative to positive charge?

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