first CV during beta: it placed a ward, you could keep 2-3 up at a time, revealed invis
2nd CV: nerfed to being able to only summon 1 ward at a time, and the mastery revealed invis
3rd CV: I don't exactly remember this one but I think they made the ward from CV reveal invis units for 30seconds or so, and the ward stayed there for 3min
4th CV: the one we all know right now
I might be wrong with those statements as it's been quite a while, but I wanted to give you the background to help you see why it doesn't reveal invis anymore.
when they nerfed the clair into an AOE spell instead of a ward they should of just left it reveal invis. that would only only allow for about 5 seconds of time to react and chase because the ability only last for 6 seconds and most times you arent standing right next to them when they go invis to run, would anyone else agree here? on top of this disadvantage only being a short AOE, the spell itself makes you more vulnerable to ganks(no flash / exhaust). i wouldnt mind the cooldown being extended if it revealed invis.
edit: also it doesnt make invis abilities usless. they are still great for initiating ganks / rushes. i personally wouldnt waste my clair if i didnt know there was an invis champion close. it just doesnt last long enough.. i mean 6 sec come on?