I think the main problem with Sneak Attack applying passively with multiple attacks is when you have multiple targets, sometimes you don't want to waste your Sneak Attack on your first attack.
Realistically speaking how often is that going to apply in practical terms?
Chances are that if you won't use your sneak attack on your first attack in the turn, you simply won't have any more chances to use it period, because "saving it for later" will mean being out of stealth.
Not to mention that at low levels more often than not your "first attack" is your ONLY attack.
It's definitely an rare scenario, but it should be common enough (especially if you're playing certain builds) that it should be considered from a design standpoint, especially later in the game as I had mentioned.
Note:
1) Advantage is only 1 of the triggers for sneak attack. Recall it also triggers when you simply have an ally engaged within 5 ft
2) Advantage doesn't always need to come from hiding - highly reliable "advantage" buffs will eventually come into play like
Greater Invisibility or Barbarian/Rogue multi-classes (Reckless Attack), which makes EVERY one of a Rogue's attack viable for sneak attack.
Sneak attacks triggering is very common - I'd argue that the need to spread / spread out your attacks is the more occasional element.
I agree 100% this isn't really needed at the lowest levels. However, based on the current pace of leveling (i.e. how fast we hit level 4), I suspect in the full game we'll blowing pass the EA cap of very early, even within the current EA content. This is why I really like the Toggle suggestion as suggested by a few other posters here - as it will scale better with the late game.
