Hmm. There's always The Game. I wonder if their manipulations used some Wraith DNA and they wound up hungering for and able to consume each others' life forces. Thus, The Game. So they had to go into stasis before they killed each other, hoping that the Asguard could find a way to not only allow them to procreate but also remove the psychopathic tendencies they'd accidentally acquired.

So the original Immortals were a bit more extreme than the Asguard in their early experimentation than the Asguard, but also realized their error sooner.

I can just imagine this explanation being given to Jack.

"So this race accidentally designed themselves to be infertile cannibal psychopaths? Wow, did they mess up."

"Yes."

"I can see why you keep them in stasis." Jack casually kicks one of the stasis pods like he would a tire on a car. "When are you going to fix them?"

"When our experiments are more successful. We were most pleased when one of our earlier experiments formed social groups. Sadly, it did not involve killing each other, but everyone else around them."

"Bummer."

"Our more recent experiments are much more successful. They form social groups and are much more likely to have a reason for any kill."

"Wow. Big improvement there." Jack is being sarcastic.

"Yes, it is."

"So where are you experimenting? Sticking them on some abandoned planet?" Jack isn't particularly interested, but figures if he can, he should probably figure out where these guys are so that he can avoid them.

"Oh no, we tried that. It did not go well. They need to be less concentrated. We found they are most successful being a minority of the population."

"Huh."

"So we put them on Earth."

"What?!!?"