On of the main reasons is political:

"It will signal that the U.S. has changed its position and no longer believes in a private-sector led internet and that governments will play a primary role in making the final decision. Russia, China, and others will welcome such a decision."

(https://www.icann.org/iana-stewardship-questions)

ICANN and others are afraid that certain, mostly authoritarian governments will want to break away from the global DNS and/or IP allocation system to form alternative, walled systems under the pretext that the US government 'controls the internet'.

So ICANN and others want to take away any argument, however feeble, that supports this position.