4) Because it was less crowded
To be honest, the royal wedding in Windsor attracted a slightly bigger crowd than the History of Cryptography and Codes. I didn’t count the people in the audience, but I am sure there were over 100. In addition, the streets around Birkbeck were considerably less crowded than usual, as most of the local people were at Windsor or watching TV.
I am glad that I attended the much smaller of the two London events that took place yesterday. While the people at Windsor had to watch a more or less boring wedding ceremony, the attendants at Birkbeck could dedicate themselved to the much more interesting topic of crypto history. Prince Harry will probably never become the king of England, but cryptology will always be the queen of sciences.
Further reading: A great event: the European Historical Ciphers Colloquium 2017