Sidelock is easier to get a better trigger release on and to regulate that pull. Also you can add intercepting sears on the "hammers" or tumblers.

It is also easier to access the strikers on a sidelock if the tip or "firing pin" part should break. On a boxlock, unless these are disc set you will have to totally strip down the gun. On a sidelock you just remove the lock plate and undo the "pin" (metal screw) that holds the hammer in place.

In terms of strength the stock of any sidelock will be stronger than any boxlock. It has to be. Less wood is removed at the head of the stock when stocking a sidelock than a boxlock and the locks themselves, being connected together across the stock by a screw, add reinforcement.

A sidelock is also better to carry broken and across the crook of the arm as it is broader where it counts. It should also weigh less and the gun should have a better "moment of inertia".

British gun makers could have made any action for their customers. It was the insistence of the customers to have the best handlong, strongest and best balanced action that caused thsoe makers to favour the sidelock. So the sidelock became pre-eminent was because it was the best solution to the customers requirements.