Because shurelia is OP, and it directly competes with Soul Shroud's itemization, and because Mana manipulator can't be gotten early cause you need wards and gp10, I liked getting it when I was newb support player, but it just isn't viable at higher levels of play.

Cause it doesn't have any items that give GP10. That's my reason, anyway... If it had Heart of Gold... I'd consider it.

I wish its passive was more like:
whenever you cast a spell upon an ally, he gets a Soul mark that lasts 3seconds. The mark triggers if your ally gets bellow 50% health and reduces all of its cooldowns by 20% based on where it's at (not the total lenght)
if a mark is triggered, Soul shroud's passive effect becomes inactive for 60 seconds.

or

Passive: Reduces the cooldown of nearby allies' summoner spells by 12%

or

Passive: Whenever one of your allies die near you, Soul shroud reduces your cooldowns by 15%

or

Active: You fire a beam similar to lux's increasing the cooldown of caught ennemy champions by 20%
for 12 seconds
60 sec. cooldown

I'd like to add to the debate that I feel the 40% maximum cooldown is a little silly considering how many interesting items can provide us with this characteristic nowadays, I would have it soft cap, granting half the amount whenever 30 is reached. It's always a big deception for newbies to learn that you can't max out this stat like you would do with attack damage or ability power.
My point about the cooldown cap really is; Not buying an item because your cooldown is maxed and you would lose in the balance really sucks, those points need to recycle into something, into cooldown for your active items, or on your summoner spells, anything but "too bad, it's lost"...

The problem with Soul Shroud is that any of the reasonable candidates for building it have more worthwhile alternatives. Simply put, it doesn't give the right combination of right amount of the right of stats to be worth buying.

They could put it back to 15% and it'd get a lot more use. Or at least bump it to 12%.

It is under rated, your mage isn't always going to have capped CDR because they aren't always going to have blue buff. Not to mention if you are running a double AP comp and have a top player like Vlad on your team it'd help both mages.

tbh i think it's cause the passive is kinda unnoticable. It's pretty neat having cdr as an aura but it'll only really apply when ur next to someone when they're casting spells which makes it kinda useless.