One of them i size/packaging. V6 is easier to adapt for front wheel duty or rear wheel duty. The V6 is easier to stuff into a front wheel drive car. One of the reasons BMW still produces I6 is because I6's are inherently smoother running than a V6 - inline 6 don't need counter balancing like so many v6's out there. Think of the forces of 3 pistons going in diagonally oposing directions - a lot of forces fighting each other there.