Short answer: we play-tested it, and it was very swingy. And messy. If you get a bad deal, there's not enough time to recover. You either destroy the game or you lose without a chance. It just didn't seem fun. Again, it was too swingy.
If you want to try a 6-Player game, go for it. You play with 4 Happenings cards and 1 Game Over card, for a total of 5 Happenings. And start on -$13.
NOTE: If you can't get a Time Travel card or a Hard Truck (or if you don't have the Warehouse Run), you will not be able to move cards from the Warehouse. And you will lose spectacularly.
There actually is some math to figure out how many Happenings cards there are. Basically, there should be 30 turns for the players. (Intermediate).
2-Player: 14 Happenings + 1 Game Over = 15 days = 15*2 = 30 player turns
3-Player: 9 Happenings + 1 Game Over = 10 days = 10*3 = 30 player turns
4-Player: 7 Happenings + 1 Game Over = 8 days = 8*4 = 32 player turns
5-Player: 5 Happenings + 1 Game Over = 6 days = 6*5 = 30 player turns
The reason the 4-player always starts at -$13 (instead of -$10 for 1-3 players) is because the players get two extra turns and a few more cards out. The 5-player starts with a lot more cards out, and even though they only get 30 plays, they are more likely to have better cards. (25 Groove cards out between the players!)
If you follow the math, then the 6-Player game would be:
6-Player: 4 Happenings + 1 Game Over = 5 days = 5*6 = 30 player turns
The worst thing would be if the first time you played, you played with 6-Players and it was either WAAAAY too easy or WAAAY too hard. It would turn you off to the game completely. So, we don't officially support the 6-Player variant. (2-5 Player games seem to go well and are pretty balanced).
So, there is a very unofficial 6-Player variant as described above. But, I wouldn't recommend it.