Precedent.
Remember, Roe vs Wade was decided almost three years after the child in question was born. It was argued that because the child had already been born, Roe would not be effectedaffected by the ruling and as such did not present a actual case or controversy. However, due to the timescales that the court works on, often years just to reach the supreme court, and then potentially years to for them to make a decision. The case fell under an established exception to the rule: one that allowed consideration of an issue that was "capable of repetition, yet evading review."
Trump's Executive order, whatever it'sits legality, is only for a relatively brief period of 120 days. This would mean that it may be impossible to review it before it is over. This could create a situation that is "capable of repetition, yet evading review."