This format was designed in order to address certain aspects of online play.
For example, it's very VERY hard to arrange that 2 specfic people be in the same place at the same time in order to do matches (oftentimes people's schedules just don't line up), but it is comparatively easier to ensure that a handful of people out of a larger group will be able to. Especially if everyone only has to remember one time and place to meet.
When online events like this get strung out over more than one day, the logistics of getting people to meet online for matches starts to catch up, the hype is lost, and these things just die.
Doing it as one big event with everyone there is also SO MUCH MORE HYPE. You wanna win with all your 'dorf brethren around, yeah?
Also, please remember this is a tournament style, not a traditional crew battle. Everyone will do 4 bo3 sets, to be matched with other players of equal w/l rank as you (swiss style). So like, if you win round 1, you'll be matched with a Marcina that also won round 1.
This allows both groups to grind out maximum MU exp with players of more or less equal skill. If you didn't know the Marcina MU before the event, afterwards, you definitely will lol.
I hope that helps address your concerns.