When you've got fountains of gold and the fruits of heavan and the worst weapons demonic smiths can devise, what use do you have for some tiny little sphere in space somewhere?
Belief.
The Kriegstanz is fought for belief.
The Blood War is fought for belief.
The Powers battle constantly for belief.
And the best belief comes from the Prime worlds. The Gods don't get any power from mortal beliefs, unless those mortals are from the prime material. Certainly, there's a reason that the Tanar'ri and Baatezu work endlessly to subvert mortals to their ends. And here's a new world. Fresh Meat!
Here are some ideas:
* A Tanar'ri prince, perhaps Orcus, decides that the new world is a prime opportunity to gain some power. On this new world, so long unmolested by Outsiders, he can set up a cult that worships him without any interference from other princes. Orcus isn't
that far from true Godhood. Perhaps a world in which he was consdiered one of the major gods could push him over the edge.
* But perhpas Graz'zt or some of the Baatezu take note of this, and send in their own forces to counter this.
* And the Harmonium gets a whiff of it too. More than any other planar faction, they're concered with the prime world, and many worlds are members of the Pax Harmonia, their world-spanning empire. They'd also play a neat opposite to the Tanar'ri and (probably) the PCs. As a lawful and (mostly) good organization, the Harmonium will do their best to stave off the chaotic evil Tanar'ri. So the PCs will probably need and want their help. On the other hand, the Harmonium is founded on the concept that the multiverse can only ever truely achieve peace and harmony when it is united under one rule - theirs. So they'll use the opportunity to try and bring the world under their rule as well (which may or may not be a bad thing depending on your view - after all, their home world of Ortho has seen peace for hundreds of years, even if it is an immensely regimented and lawful place).
* Perhaps the Gith, mistakenly or not, believe that this may be their long-lost homeworld, prompting the Githyanki and Githzeari to escelate their skirmishes into full-blown war over the place.
Anyway, I hope this helps.