Reasons why Congress must do this:

• Any conviction for a crime committed in this new Indian Territory may be thrown out. The Solicitor General of Oklahoma said that there are 1700 criminals that may be released.
• Properties sold and bought under the jurisdiction of the State of Oklahoma may not be recognized, since those laws do not apply.
• It is unclear (to me) whether the residents of this territory will have representation in Congress. If Congress does its job, they will.
• The Creek nation says that they will work with the State of Oklahoma. However, there are other territories in the United States that could be given back, and those tribes could be less willing to cooperate.

There are many more reasons, and I will add them here as time goes on.