Porting has a reputation for utterly destroying the value of a fine arm. Doing so to a double rifle would easily remove $10k from the resale value, so most people wouldn't do it just for that reason alone.
On a technical level, the regulation comes into play in large part within the last 3" of the barrel via the regulating wedge. It would really alter regulation a LOT. Lastly, you cannot symetrically port a double barrel rifle because of the ribs. So you're creating asymetric recoil and regulation issues.
