Because although the server can handle a tremendous number of players, It can’t handle a tremendous number of players all in one location. The nature of data transfer between client and server is multiplicative in nature and scales exponentially as more and more data has to be communicated. This has been shown many times recently, the servers will not handle hundreds of players in the same place at the same time.
Which means Blizzard will have to drastically lower the number of people logging into the server at the same time; they will need to enable a queue (without sharding).