Conduits under walls break after turret or grenade explosion, while conduits in walls just get damaged. I'm sure it's the reason to have conduit walls behind turrets. Of course you still able to protect conduits with additional wall, but in cost of materials, place and sometimes beauty.
It also affected beauty stats in alpha9 - conduits built over walls decreased beauty, while conduits hidden inside did not - makes sense for me. It also gave you options - you can make fast connection in cost of safety and beauty or rebuild walls in cost of time.
It can be modded back though, but I just think conduit walls deserve to be there by default because of foresaid reasons.