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.
