Rogue traditionally get backstab dmg since a long time. Its make sense that you can do a lethal strike easily from the back, but that I'd be impossible from the front (because the ennemy is defending, or see it coming). Its also make sense that its harder to deal a critical from the side than from the back, because the opponent can still see you with his peripheral vision if you re flanking him, but he is defenceless if you go from behind.
Nevertheless, I personnally don't mind a flanking advantage as long as it not op. Just my 2 cents.
Just FYI, that is not correct.
Rogues in 5e get "Sneak Attack" damage, which happens when they make an attack at Advantage or when an Ally is within 5 feet of the target. Having Disadvantage on the attack roll removes all Sneak Attack options.
That rule is partially why Rogues exclusively have Cunning Action Stealth and move. Giving everyone that class feature cheapened the value of a Rogue significantly.
Facing does not exist for all practicality in 5e, and that is a good thing.
