There's more to it than the rubber hoods.

Long-cable-pull (LP) levers are capable of pulling more cable with about the same lever movement by moving the fulcrum point lower on the lever arm. You have to reach further out and down to actuate a LP lever, and you have to apply more leverage. LP levers have less mechanical advantage than std road levers.

A standard short-cable-pull road lever, whether plain ole brake lever or modern integrated brake/shift lever aka brifter (STI/Ergo) is simply easier to reach, wrap your fingers around, and pull, requiring less effort due to it's greater leverage/mechanical advantage.

Another thought - perhaps Surly went with Avid BB7 MTB calipers because a certain number of customers remove the drop bars and switch to flat, riser or trekking/butterfly bars - all of which require mtb brake levers, which subsequently require BB7M calipers - not BB7 Road.

Or it's as simple as the mtb calipers mean another dollar profit plus it keeps all the parts black.