That’s where people are.
The number of people hanging out on some government website is probably low.
The person tasked with warning is probably concerned with getting the word out to as many people as possible and less so how we would like things to be (not have to use twitter).
If this was only announced via some obscure website we would be wondering why they didn't just tweet it out / why the government would expect people to just find some random website and refresh it all day.