The PHB likely says "can" because it's allowing for cases where you can't, such as having disadvantage, or there not being one of your allies within range to make your attack qualify for Sneak Attack. Which of those conditions cancels out Colossus Slayer? This isn't something that's toggled, it has rules that apply to when it can and cannot apply. When those conditions are met, it's a passive bonus to your attack, when they're not, they don't apply, hence "can".
Sneak attack should NEVER be passive. Read your own quote of the PHB... once per turn (when you meet the conditions) you CAN deal an extra... can makes it a choice and especially when one has multiple attacks it should be a choice and not just something that happens.
I don't need to? You don't have to roll a special attack for that damage, it is applied on a successful attack, where the conditions are met, hence it is, in fact, passive. You don't roll a "Sneak Attack" with your crossbow, when an enemy is engaged with one of your allies, your roll an attack roll, and since the target is engaged with an ally, you get your sneak attack bonus damage. If you attack from stealth, you also do not roll a "Sneak Attack", but just your regular attack, and the bonus damage is applied, if you hit. Sounds pretty passive to me. Even the way the die for that damage is acquired is passive, since it accrues as the character levels, no special feats or skills that require you to choose them, they are added at the appropriate class levels, for free.
