User blog comment:FairyRaven/Fairy Tail Chapter 407: All for My Demise, Review and Commentary/@comment-25334245-20141101183131/@comment-7418318-20141101215426

Maybe it's effect is stronger the closer you are to it. With Erza already being as overpowered as she is, a little boost was enough for her, but the others might well need the full dose... so to say.

Or maybe activating LH required a ritual that either needs many people to perform or becomes more powerful if more people participate. Considering how Mavis used everyone's energy to cast Fairy Sphere back on Tenrou, I wouldn't be surprised if channeling the energy of as many FT members as possible is part of whatever LH does.