Despite its namesake, Sneak Attack is not an actual attack. It is a feature that allows Rogues to do extra damage should its conditions be met. The flaw in the way it is currently implemented shows itself if you are dual wielding. You use the Sneak Attack feature as an Action, and if you miss, you're left with your bonus action attack, which will not apply your Sneak Attack should you hit. This is not how Sneak Attack works in 5e. You just attack using one weapon, and if you miss, then you can still use your bonus action to attack with your other weapon, and you get to apply your Sneak Attack damage should you hit with that one instead.
Making Sneak Attack passive allows for Attack, Extra Attack, Bonus Action Attack, Opportunity Attack, and if they implement them spells like Booming Blade and Green-Flame Blade the ability to apply Sneak Attack damage if the conditions are met.
