My guess would be because the Netherrealm is the more powerful of the realms, and the only one I believe he could successfully barter with. Apparently Quan Chi has the authority of the Netherrealm, but do we even know who rules the others? Chaosrealm would be leaderless, and an alliance likely futile. Orderrealm would like refuse because allying with Earthrealm during all its chaos would breed disorder for them.

So... Netherrealm it is.

I like how Raiden already knows Quan is working for Kahn because Quan and Noob were behind the soulnado Nightwolf stopped...and he still thinks the Netherealm is a neutral entity he can get to side with him.

I would've said because the writers have realized how retarded Orderrealm and Chaosrealm are...but they're mentioned in the goddamned endings.

So I'll say...because they're setting up for the MK4 remake next game.

He just got too desperate. And Orrderealm would probably have to arrange a court date for Raiden before he could apply for assistance.

I think it's because raiden has a thing for bald headed pale men with the power to manipulate mortals.

HAHAHAHAHA

Because the story is absolutely coconuts!

Coconuts I tell ya!

Raiden's not nuts, and this is not some enormous plothole. The Netherealm is a logical choice.

Quan Chi is behind the Soulnado, behind Sindel's soul being tainted & resurrected, etc. In fact a lot of why Shao Kahn seems so powerful is really because of events put into action by Quan Chi.

Raiden has already turned several bad guys into good ones. (Remember Johnny's line, "What's he got that I haven't?") If he can turn Quan Chi, Earth realm doesn't just get stronger, but Outworld gets weaker.

But he didnt really try and turn Quan Chi good.

He was willing to accept the gravest price for his help.

He was planning on selling out all the dead heroes from Earthrealm, so he thought he had a bargaining chip to get Quan Chi to switch sides.

It apparently never occured to him that Shao Kahn could have offered Quan Chi that very same bargaining chip (of course, it also didn't occur to Raiden that Quan Chi wasn't actually working for Shao Kahn in the first place and was just playing everyone against each other for Shinnok's benefit).

because adding those other realms would require adding more characters

Because he is as desperate as he can be at that moment and its probably the first and only solution that comes to his mind, given that the Netherealm is the only realm that could possibly be a match against Outworld.

And given that Quan Chi doesn't care about allegiances or remaining true to his word, as long as he has the bargain that benefits him the most, Raiden probably thought that his offer had a slim chance of persuading him into aiding him and turning against Kahn. He also clearly overestimated the value of his own soul.

What I wonder, though, shouldn't Shinnok be the one calling the shots in the Netherealm around this time? And shouldn't Raiden be aware of the fact that Shinnok is rising in power in the Netherealm? Cause if he knew this, he should have also known that Shinnok hates Raiden's guts more than Shao Kahn.

Y'know maybe it's just me, but personally, I think Quan Chi owning my soul would be a fate much worse than Shao Kahn winning. I mean, what's the worst Kahn can do to a person, really? If he takes your soul, you just become a source of energy. If you're in the Netherealm, you're a goddamn zombie slave.