It makes sense from a design perspective. If you had those two cannons pointing out the front, you wouldn't be able to put the driver's position there. Meaning you'd either have to elevate the drivers position, exposing him to fire, or use some some expensive and fiddly sensor array in order for him to be able to see where the hell he was going. It's really a very sensible design, insofar as anything in 40k can be called sensible.