We do this as we have limited storage space in our stores. Some days, all bikes in the stores are booked and we try our best to minimize driving bikes around and make sure no one has to wait. When you pre-order, you can only choose a pickup first the next day.
During high season (and when semesters start), pickup slots might get a bit booked up.
If you want a bike the same day of ordering, you can always pop by before closing time and take a chance to see if any are left.