They don’t want to risk a Supreme Court smackdown.
Which results in the appearance of some 3-judge panels with 2 conservative judges. Which means this isn't over. A replay before a larger panel might have a liberal majority that time, and this decision would be reversed. That depends on appellate practice strategic choices by the plaintiffs here and the 9th Circuit which can't be determined right now. Trump's appointment of so many new Circuit Court judges creates too much uncertainty to call that one.