The exact reason for him inviting the best fighters have been cited or guesstimated as follows:

• Shang Tsung needs warrior's souls to stay young and/or keep the portal to from Outworld to Earth open (Not just any old soul will do!)
  
• The Elder Gods wouldn't count Outworld's victory as a true victory against Earth unless the best warriors were there to represent Earth
  
• He may have been under the impression that someone like Goro could never lose, so he has nothing to lose playing by the rules and inviting the strongest Earth has to offer.

Some of this is conjecture, but I hope these explanations help!

Also, he may have wanted a challenge.

maybe Raiden knew who were really the best fighters anyway so Tsung could not trick him

Maybe he thought those warriors could be corrupted somehow?

The rules say MK has to be fought by the best of the realm. As grandmaster, Shang has a lot of leeway to change the rules and make up his own, but if he actually cheated like that to give Outworld a real handicap, the gods would probably call bullshit.