Same reason everything else is. Trying to keep a nasty-ass disease from getting deeper into the general population. Until there's a vaccine or other treatment found and widely distributed, any large gathering is just asking to start a fresh wave of infections.
That be true, yes. Almost every other con has been cancelled or, at the very least, postponed so far. Things are going to work out and probably sooner than we think.
even a few events here got cancelled
Like the annual car show