During the light cycle your plant produces a hormone that suppresses flowering. This hormone is slowly depleted during the night cycle. When it reaches a low enough level your plant starts to flower. By having a long dark cycle you remove the hormone faster and reduce the flowing time period needed for a finish product.
I've heard this and variations on it many times before. I'm curious if anyone has a legitimate scientific paper or research article to corroborate this in cannabis plants, though, because I've never seen it mentioned in any way other than anecdotally.