Let $D$ a division ring. Why $D^n$ needs to be a right $D$-vector space?

Question

Let $D$ be a division ring then take $M_n(D)$ the matrix ring over $D$. Now we have that $D^n$ the only simple left $M_n(D)$-module. During the proof some authors (e.g. Lam in "A first course in non-commutative algebra"), note the fact that we have to take $D^n$ as a right $D$-vector space. I don't see why.
Any help? Thanks.