Well, recently in the Boom Comics, I believe they explained that Zordon hid the Zeo Crystal on Earth because they needed some out of the way, backwater place no one would suspect. Mind you, the Zeo Crystal seems to have a VERY different backstory and abilities in the Boom Comics, which makes it much more important. Not only is it the source of Zedd's origin, apparently it was created when a space god killed a space demon. And the Zeo Crystal was never actually destroyed or lost or anything. It's still somewhere on Earth. So...it'd be pretty easy to say that maybe the Zeo Crystal is making Earth more "radioactive" with higher than normal Morphin energy levels.
Or, it would be pretty easy to tie most of this in with Zordon. Most of the early enemies only seemed to come to Earth because Zordon was here and they wanted him dead. Zordon not only lived on Earth for hundreds/thousands of years, he also died there, and he left different backup plans like Gosei from Megaforce on Earth. It wouldn't be too hard to say that Zordon's spirit or different backup plans might still be nudging things here or there to make it easier for humans to rediscover the Morphin Grid over and over again. Or maybe Zordon's death or the many Ranger teams in the past actually has made Earth more "active" with Morphin energy. Also, Earth is the place were Dark Specter "died", and given in the comics he's now the cosmic embodiment of the Dark Side of the Force, that might further add to Earth's Morphin radioactivity.
Or maybe the Morphin Grid is just rediculously easy to access once you know it's there. People on Earth have known about Ranger teams for decades now. It might just be natural to find this stuff, once you kind of know what to look for. Once you know you can make fire by banging on some rocks, it's pretty easy to just keep banging rocks together until you find the right type of rocks, after all.
Or, you could always just use the convenient excuse that Earth is just at a nexus of some big, cosmic Morphin ley lines. That it's just naturally, coincidentally connected to Morphin energy by being in the right place at the right time.
Or, maybe the Morphin Grid just likes humans. Like the Doctor from Doctor Who just liking Earthlings. Like it or not, we've seen in recent years that the Morphin Grid has...some sort of intelligence behind it. Giving people visions of the future (and maybe explaining why some people like Tommy get preferential treatment). Maybe the Morphin Grid just thinks we're neat.
Or, maybe Earth isn't really all that special after all. We've seen other planets with Power Rangers like the Aquitarians, after all. Maybe Earth just has so many Ranger teams because we have short attention spans and just can't commit to a theme. As many teams as Earth has had, there's usually only one team active at any one time after all.
For a long time, my personal headcanon was that Zordon basically MADE the Morphin Grid. This...is no longer possible due to newer canon sources. But, the term "Grid" always made me think of something that was artificial. I don't think many things in nature actually come out as a "grid". I supposed you could still say that the Morphin Masters made the actual "Grid" part for the Morphin energy to flow through. Maybe this natutally gave rise to the light and Dark Specter, as some sort of artificial intelligences. The Morphin Masters might have given the energy of the universe the ability to perceive itself by creating a cosmic "brain" for it to think with. But, this is all fanon.
Any way you do this, you might want to keep a eye on the Power Rangers Universe miniseries right now. I have a feeling this isn't just exploring the origins of the Morphin Masters, but maybe the Emissaries and the Empyreals, too. There's 3 Emissaries and 3 Empyreals, and there's just coincidentally 6 "original Rangers" who can access any Ranger suit from across time...maybe I'm jumping the gun, but it seems like there's a connection there.