It's mostly an accident of circumstance.
The Powers That Be had a long play that no one knew about. It culminated in the finale of Wizards at War, but began thousands of years prior - as far as we know, the first move being the Power that chose to stay in Ireland:
"He knew, though. That much I gathered as He was leaving. And He was just after gathering that the Spear hadn't been forged for me, or for Him, all that while ago. It was always forged for Her, for the Hesper, even when the Smith of Falias first made it, ten thousand years ago. And I thought I was just a spear-carrier? So was He." And Ronan laughed, then. "He thought it was a stitch. You should have heard Him laughing."
"I did. I mentioned about His sense of humour," Nita said, and rubbed one ear in memory of having been bitten there by the Defender, long ago.
"You did. But the whole bloody thing was a setup. The Hesper'd never have broken loose all the way unless the Lone One was there trying to stop Her. If It'd just ignored Her, none of this would ever have happened." He grinned that dark grin of his. "But wasn't that how They had it planned?"
Tracking backwards, the incarnation of the Defender that Nita is remembering is the macaw named Machu Picchu, back in High Wizardry. Two major things happened in that book:
- When the mobiles were created, this was the first time in all of history that the Lone Power totally and completely lost a Choice.
- Dairine's raw power, combined with the information dump of the entire manual into her mind, allowed her to temporarily overpower the Lone Power. This gave the Defender the opportunity, and finally redeemed the Lone Power.
- The Defender also explicitly states that this was the first time they could do anything permanent to the Lone Power. They needed what Dairine did.
But that wasn't the only role of mobiles that Dairine created in High Wizardry. Because they totally rejected the Choice, the Lone Power had no hold on them as it did all other life in the universe. This made them an ideal method of transporting information by the other Powers That Be, since the Lone Power had no way of listening in on them. Come Wizards at War, they were the ones that decoded the message left in Spot, and let the main characters even know the Hesper existed.
Going back a bit further, since with High Wizardry we know Machu Picchu was actually the Defender, his advice in So You Want to Be a Wizard "Don't be afraid to make corrections." to Nita has a bit more meaning. At the time it seemed a bit random, but it was why Nita changed the Lone Power's name at the end of that book. Changing a name in the Speech actually changes what it represents; before Nita's change, the Lone Power's redemption in High Wizardry would not have been possible.
But that's not all! Something I think you missed, is that the Powers exist outside of Time, including the Lone Power. They have an innumerable number of incarnations and avatars, several of which we have seen existing at the same time:
Machu Picchu and Ronan both housed the Defender, although Ronan was not aware of it until after Machu Picchu died.
The Lone Power existed, trapped, on the Alaalid world in Wizard's Holiday for millenia while simultaneously existing as the antagonists in all the other books.
The entire Yaldiv species was so totally lost that they were all individual avatars of the Lone Power.
The Lone Power is known on Filif's planet as the "Kindler of Wildfires".
Aside from these characters, it's stated explicitly twice by the Lone Power itself. First in High Wizardry when it was redeemed:
It spoke to her, not with words but as if she were thinking to herself. My shadows are still abroad in the world. As I have done evil, for some time yet they still shall. Stop them.. Stop me.
And secondly when one of its incarnations was complaining in Wizard's Holiday:
Nita threw Kit a glance. The Lone One sat there for a moment, drumming Its fingers on the arm of Its severely plain throne. "You know how the shift in me happened, a while ago," the Lone One said. "You two and Dairine were simply party to a change of nature that the Powers That Be and just about all of creation had been pushing on me for aeons... slowly wearing me down until the last big push came. You just happened to be part of the breakthrough, part of the point of the spear. Because you live in Time, it looks to you like that was a thing that happened then and was over with, whereas outside of Time, the event both happened aeons ago and is still happening."
So the whole focus-on-Earth thing tracks back to two events that might be related to each other, and one point-of-view issue:
- The Power that created the Spear for the Hesper did it while on Earth, and decided to stay. This then required the Defender to later get an avatar on that planet so that it could obtain the Spear for delivery. Although the Defender didn't know it was to deliver the spear, he believed the spear was meant for him; everything the Spear was used for prior to the Hesper involved Ireland, so his avatar being a human from Ireland simply makes sense.
- Nita and Dairine together formed the final push towards redemption for the Lone Power; they could have been any species, except that as a side-effect, the mobiles created during the event were able to pass along information in a way the Lone Power couldn't overhear. Because the plan already involved humans by way of Ronan, getting him in touch with the mobiles would be easier if other humans were involved.
- Purely speculation on my part, but Nita and Dairine's family is from Ireland. Maybe that also had some influence on why it was them who helped redeem the Lone Power?
- The Lone Power is everywhere, but we do see it interacting with humans most often because not only are these humans rather important, most of the main characters are human anyway.