Blizzard made the final decision to cancel BlizzCon 2020 on Tuesday amid the COVID-19 pandemic.
The reason cited in the letter posted on the BlizzCon website is health and safety concerns. Specifically the letter cites "fluctuations in national and local health guidelines" as a major concern.
These concerns are based on the the fact that not only could the number of people who could attend be limited, attendees that have to travel to the venue from far away pose a public health risk, and attendees could face problems travelling in general like cancelled flights due to empty seats.
BlizzCon 2020 Cancelled Amid COVID-19 Pandemic
Although it is unfortunate that there will not be any exclusive cosmetics or game announcements, like an Overwatch 2 beta, Blizzard hasn't forgotten about players. They are currently looking into hosting an online event, but it will most likely happen in early 2021.
Although BlizzCon has had virtual tickets before, the event was directed towards attendees, so switching to an entirely digital event will be challenging. The letter specifically mentions big esports events as a core part of BlizzCon that they will try to preserve in a different format at some point in the future.