> why is it Zoom that is blowing up because of this and not any of the other companies?

I suspect it seems that way because Zoom was already more aggressively seeking media spotlight as a growth startup, and experiencing more rapid growth in terms of multiples because it had a much smaller install base to start with than established competitors. Also, Zoom is the center of it's company’s business whereas Slack and Webex are just part of a large stable for their respective firms.