It's an issue with design complexity. Even though sneak attack would be limited to once per turn, were rogues to have multiple attacks per round, that combination of rules is much more complex than the alternative. As long as rogues only have one attack, you can simply explain that rogues get sneak attack as long as they meet the conditions, and you don't have to worry about the per-turn limit. If someone wants to muck about with two-weapon fighting or something like that, then they can deal with that as an exception to how rogues normally work, and normal players never have to worry about it.While looking over the classes, it occured to me that Rogues are the only "battler" class (Barbarian, Fighter, Paladin, Monk, Ranger, and Rogue) that don't every get the Extra Attack feature. The other five classes all get it at 5th level. Is there some reason behind this? They are limited to one sneak attack per turn, so it isn't as though they could get more sneak attacks or something.
You see the same issue with clerics, who only get their bonus damage to one attack per round. They aren't going to impose a limit where they can be reasonably certain that you'll bump up against that limit on a regular basis. If you want to find some way for them to get extra attacks, then that's on you, and the once-per-turn rule is only there to make sure you don't accidentally break something by doing so.