There are 3 reasons that Redemption/Locket is the best combination for supports

and probably will be even if the items are nerfed repeatedly; they're AoE, easy to use, and easy to appreciate.

When a large chunk of support items only allow you to provide benefit to one person at a time (and it can be a difficult decision to make with how fast teamfights are), it's no wonder that the two items that provide immense defensive value to your entire team are miles better than their competition unless their competition gets buffed heavily to compensate for not being team-wide effects.The only other support item that sees play often is because it has direct synergy with these two. (and locket and redemption have synergy with each other)

are all items that only give you boosts to one ally, and sometimes yourself. more importantly, it's hard to coordinate three of these items to maximum effect in soloQ; Zeke's activating is largely out of your control and can pop just off of trying to clear minion waves, Mikael's means that your ADC both depends on you to use it at the right time AND for that ADC to notice it so they can avoid eating another one. And Knight's Vow requires you to have someone worth binding to that can make the most of it (ADCs are not usually worth binding to). Redemption and Locket, on the other hand, are as straightforward as can be; you don't need to coordinate anything to just drop a heal or a fat shield on your team, it's practically fire and forget. Items that don't feel like you rely on your teammates to make your actives matter (or at least, having some more control over those actives yourself ) would go a long way, I bet.

Lastly the issue of satisfaction; even if these items are used well, it's hard to appreciate it. Look at Redemption's active; it's big, the heal pop is loud, it's NOTICEABLE and it feels good to use. This is the easiest fix; just make these other items more noticeable too.

make support great again