Why is Guardian the only ROLE-EXCLUSIVE keystone?

PASSIVE: While within 175 units of an allied champion, you raise your Guard. Allied champions you affect with unit-targeted abilities are also Guarded for 2.5 seconds, irrelevant of distance. If you or a Guarded ally would take damage from an enemy champion or monster, you both first gain a shield for 70 - 150 (based on level) (+ 25% Guardian's AP) (+ 12% Guardian's bonus health) as well as 20% bonus Movement speed icon movement speed for 1.5 seconds. Guardian only goes on cooldown when the shield is triggered. Cooldown 70 - 40 (based on level) seconds

Take this top or mid? .. You have no allied champions to be near or to affect with uni-targeted abilities, .. unless they are roaming or ganking.

Take this Jungle and you can only use it while ganking.

That leaves ADC and support as the only Duo-lane. Technically an ADC could take this, but there are plenty of other keystones that would honestly be better.

HP tanks / AP supports are really THE ONLY role that can use this keystone effectively.

Why is that?

Could this be changed so that it actually makes sense for other roles?

What if this were changed so that you are simply "Guarded" while it is off of cooldown, regardless of your proximity to an allied champion? If you would receive damage the shield and movement speed is proc'd. Also, increase the base cooldown, but have the cooldown decrease faster while near an allied champion.

This gives the keystone functionality in all roles, but still encourages supports to take it.

Also, it gives the enemy laners the ability to trigger the shield at a time when the support is not guarding the carry.