The Locket isn't a bad item. It's just that, with the items that are required to make the item, you may as well buy Randuin's Omen.
Randuin's Omen is more geared towards tanks and bruisers, so the on-use active ability works in their favor when they're in the middle of a team fight. However, the Locket, as an item that I feel is primarily directed towards a support to pick up, is counter intuitive for the role of the support because of it's active ability; the support has to be within range of everyone on his team to have the shield be put on everyone, and by then, you're in a very dangerous spot. Of course, there are some champions like Taric and Leona that can afford to do this, but not for the other support champions.
There are several things that may boost the popularity of this item, whether it be individual options or a combination of all of them:
1) Include a 5% CDR
2) Make the active on-use select a region. Something parallel to how Leona's ultimate works. Or Vlad's ultimate. That way, the support can put the shield over his teammates and still remain in a good position.
3) Increase scaling to +15 per level instead of +10
I'm all in favor of Option #2. I feel the Locket is a good item; any actual number buffs to the item would probably cause an increase in the price as well. However, Option #2 is more of a QoL change to make it more attractive for supports to pick up.