Think of what happens when you lower or raise the SRV, the wheels rotate and retract into a stowed position. To me that would mean that there isn't a lot of free space around the SRV when it is in the vehicle hangar which is why boarding the SRV via the front makes sense.
I would suggest that the side door initially had some inward or outward movement (like an aircraft door) then slid backward, inward or outward of the cockpit bubble to open...not that difficult an action considering it's already current and widely used aerospace technology from commercial aircraft to the space shuttle.
For those who would suggest the seat would be an obstacle, I'm sure that a rotating seat would be far less complicated than the entire cockpit and all the ancillary equipment having to slide up, down and sideways to be less of an obstruction to exit is just plain over complication for the sake of looking cool. It's not simply that so many, really unnecessarily, moving parts creates it's own potential mechanical nightmare...but the electrical side is another pitfall. In a mechanical or design perspective...I was always taught K.I.S.S (Keep It Simple, Stupid)
I'm no design engineer, simply an ex tank crewman and practical mechanic who would shudder at the thought of a piece of equipment being designed to look cool rather than just plainly...and simply... being functional or practical.