It builds up tension for starters, if one team is 2-0, the third game is very important for the team without a win, and puts some pressure on the player, as well as makes it more exciting. If you do 3 games at once, you'll finish watching the first game, and the commentator will announce 'Oh yea, and X and Y won so team Z has already won' after only seeing a few maps.
Second, split viewership, lets say you have 500 concurrent viewers watching (Random number), if you stream 3 games at once, you're going to have something like 200/200/100 (Random numbers) and each stream is worse off because of it. Also, theres quite a large turnover on viewers, a few people will tune in late or tune out early for a variety of reasons. Lets say you get 1500 stream views in an hour, splitting the event up and doing 3 streams means you'll be casting for about an hour to an hour and a half, giving you about 2k views total, but if you have 1 guy run it, it'll last 3-4 hours and you'll have 4500-6000 views total, which is more impressive to sponsors obviously.
Finally, running the event for 3-4 hours makes it more of an event, if it only lasts an hour its more of an afterthought and people are more prone to missing it.
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