Poe called the big dreadnought a 'fleet-killer', and it was targeting the rebels flagship before it got blown up overdramaticaly by the last dying bomber.
Unlike the (turbo)lasers from Snokes ship their main cannon would have punched through the shilds and taken them down.
Hell, why did the FO not simply order a few of the accompanying Star Destroyers to jump ahead and cut the Rebels off? This part (18h long car chase) of the plot makes no sense whatsoever even by SW standards.