When a popular event goes on sale there are literally hundreds and sometimes thousands of customers simultaneously attempting to purchase tickets across all distribution channels. Like any limited commodity, tickets are sold as long as seats are available, and sometimes they go quickly. If 500 fans are simultaneously buying four tickets each, 2,000 tickets can be sold in a matter of seconds! It's the same reason why a typical 18,000-seat arena can sell out in a few minutes.
Also, in many cases, tickets are held by the promoter or venue or have been pre-sold to season ticket holders for the entire run of the event reducing the quantity of tickets available for purchase through Ticketmaster by the general public.