Three problems you aren't considering.
First, Challenger Series takes place already on Wednesday and Thursday. No, they aren't as big, but that's why they are on less optimal days of the week and LCS gets prime weekend time for fans to enjoy.
Second, Riot has two tricked out studios for the broadcasts. Why make one worthless NOW after it being all set up? That's just a waste of resources.
Third, time.
If six games were played all in one day, when would broadcast start? Noon EST? That means everyone in studio has to be in and be ready for a 9am start. Casters, players, production, spectators.
On the other end, if all the games run long that day, the broadcast is going until around Midnight on the east coast.
How many people would REALLY watch a 12 hour marathon EVERY WEEK? Even assuming there are a lot of lifeless NEETs, they will not be giving that much undivided attention. They will be playing their own matches or doing other things.
The format is fine as is.