Because 4 years is a short time. At this point, the organizers need concrete solutions, not abstract goals. They need a good enough solution that is guaranteed

Because 4 years is a short time. At this point, the organizers need concrete solutions, not abstract goals. They need a good enough solution that is guaranteed to work on the specific dates, not something potentially better but uncertain and at a risk of being delayed. If you have a mass event and most people arrive by car, you need much more space for parking than for the event itself. If you don't have that space, you need strong incentives against driving. The last thing you want is more people arriving by car than there is parking space. Street capacity can also be an issue. If you have tens of thousands of people arriving/leaving a venue at the same time, repeated several times around the city, you don't want to waste capacity on small vehicles carrying only a handful of passengers. You load the people in public transit, drop them off anywhere else, and let them figure out how to reach their actual destinations from there.