 Why? Because if they do it when Trump is president, he might do something about it. They believe they'll be able to wheedle a better deal out of Joe Biden than they will be able to wheedle out of Donald Trump. Donald Trump reacts to threat with aggressiveness. Joe Biden reacts to threat with cowardice, and they know this. So if they have a choice between blockading Taiwan while Biden is president or blockading Taiwan while Trump is president, and they're already certain that Biden's going to lose.