If I remember right, Trump tried to appeal directly to SCOTUS off the bat, correct? My guess is that Roberts wanted the weight of having district and appellate court opinions on his side before making a judgement. He cares about process, and he was probably terrified that a rushed judgement would just create more chaos, especially when Thomas and probably Alito voted in favor of Trump. They are less likely to go against the majority of the Court now that they would have to refute two absolutely bulletproof opinions. My guess is that Roberts really, really wants this case to be about the President, and not Trump per se. And in that case, it's his judgement that it should be considered with the full weight of their normal process without reference to what's happening in the world at this precise moment. Just a guess, but I think probably a good one.