Simple villain arrogance. He figured Scorpion wasn't powerful enough to defeat him and since Sub-Zero was (almost) out of the picture, he decided he no longer needed him.

I thought it was more stupid that Scorpion bought Quan Chi's lies about the younger Sub-Zero wholesale in MK4 and jump-started his quest for vengeance.

He probably just wanted to take sadistic pleasure in notifying Scorpion that he himself had massacred his entire family and clan, and easily tricked him into hunting down the wrong man.
Plus, he figured Sub-Zero was about to die, and he probably would have disposed of Scorpion later. He immediately tried to send Scorpion back to the Netherrealm, but got yanked into the portal with him.

Scorpion is a dog without a leash in almost every instance. Someone just has to point him in the right direction and he attacks. I am in agreement with JediSith about Quan Chi underestimating Scorpion.

It's kind of like a parent and child relationship. Quan Chi created Scorpion, and most likely sees him inferior. Quan Chi deceived and defeated a god, Scorpion should be no match in his mind.

Both Scorpion and Quan Chi were stupid in MK4, but Quan Chi was a pretty smart guy in Mythologies. Sending both the elder Sub and Scorpion at the same time for that map was a smart move, and he got Scorpion's soul out of the deal (at least in the reboot he does; Scorpion's his lapdog).

and just like every villain in this series:

Yeah, that was pretty dumb. But Quan Chi is still smarter and more cunning than any other of the villains from the MK universe. Doesn't help that the rest of them are colossal retards, but in this universe, you take what you can get right?

I don't know I think Shang is pretty cunning. How the fuck do you get all of the heroes to follow you into a trap in outworld?

He may be stupid, but that shinnok is even dumber.

*looks at the question in the title and answers...*

Yes.

Like JediSith said, arrogance

Confidence comes into play as well. Its stated in MKDA's konquest mode that Quan Chi fights and defeats Scorpion at one point after the transport to the Netherrealm if I am correct.

Quan is no panzy.

I never said that the heroes of MK where that much smarter. And Shang Tsung is completely incompetent. He can't do anything right on his own, even with all power Kahn granted him.

Yeah, it's pretty stupid. Quan Chi was either too slow or he wasn't paying attention like he should have. Also, to make things worse, the way Scorpion's ending ended had Scorpion standing right next to Quan Chi while he keeps screaming "NOOOOOOOOOOOO!". Gotta love those cheesy cutscenes.

MK2 wasn't really a "trap".

I mean, it was, but it was an obvious one that the heroes knew about and willingly walked into.

They knew they were fighting the Outworlders on their home turf, they knew Shao Kahn would be the final battle, they knew everything about MK2 (except for the fact that it was secretly just a distraction so they wouldn't notice Sindel's resurrection)

...and they went anyway.

They knew there was no logical reason to accept the challenge. They were under no obligation, they had already won the first tournament, they didn't have to agree to MK2, they could've just said "Nope, you lost, get off our planet and don't come back until 50 years from now at the next real tourney."

...and they went anyway.

WHY did the heroes agree to MK2? Two reasons: To get revenge for the Shaolin slaughter, and to rescue Sonya. That's just ordinary baiting. "Piss them off so bad they say yes to a very stupid idea" is not that clever.

Wait. He thought he needed the power of Scorpion to beat "his nemesis" Sub-Zero, so he's pretty much admitting himself weaker than them.

Thanks for explaining that better. That actually makes sense. But if they where going because of the shaolin slaughter and to rescue Sonya, then what the hell was Shang doing?

I always imagine Khan saying "Alrite I gave you your youth back now go get those heroes here." Then Shang just happens to bump into the good guys and give them the finger and so they foolishly chase after him into a suspicious portal. I'm really anxious to see how this plays out in storymode. (please no spoilers).

Well given younger Sub's personality and Scorpion's past with him, plus how much of a creep Quan Chi is, its not too farfetched to believe him.

But yeah, Quan Chi is stupid. Here's how it should have gone down:

Quan: I killed you family and clan and now I'll send you to the neatherrealm!
Scorp: I'll stop you!
Quan: I'm not a Saturday morning cartoon villain Dan. I sent you to hell 35 minutes ago!

Shang Tsung is really a brainiac henchman, rather than a manipulator and schemer like Quan Chi. Shang's only trickery of which I know is the one he played with Kenshi when leading him to the well. He can be assigned to imbalance the furies or revive Sindel, but those are his tasks, not something he thought of first, independently.

Then again, Shang's position of a servant, as well as his complex obligations, weren't that likely to give him freedom to make plans of overthrowing Kahn. True, Quan Chi was Shinnok's arch sorcerer, too, but he never really gave an impression of being Shinnok's lowly stooge, more so taking into account he regarded Shinnok as his debtor, while Kahn never owed anything to Shang Tsung.

Guess that is to explain the source of Quan Chi's apparent intellectual superiority.