It could have been poorly marketed, but look at the time of year. People are on vacation, at their cottage and watching hockey in the summer is probably not on the top of their list. Even so, surprisingly there were people complaining that the SCF went far too late into the early summer.
Now, as an aside, I think even if it was somewhat marketed well, many people won't tune in because it's not the truly "best of the best", almost at NHL level or NHL level players, unlike the World Juniors, which happens during the hockey season, during the winter at a time where people are off and sitting at home with friends and family. People are already in a hockey spectator mindset then. I doubt it is at the same level in the middle of the summer.
Perhaps there is even a novelty component that wears off over time. Like all the outdoor games, but those still feature full roster NHL players, and fans look to see their favourites there.