There are two reasons: the item you purchased is out of stock; the item you purchased will need 10-15 business days to be prepared.
As you know, SHEIN is a large on-line shop, which has a large number of orders every day. Our warehouse staff will process those parcels one by one in chronological order.
When you place an order, it may show that one item is still in stock. But in fact, the item was pre-ordered by other previous orders and may be out-of-stock when we process your order.