Let's be honest, Stake.com cannot celebrate all of these "special" days, because they will probably go bankrupt if they do. Some of these days have zero marketing potential for the casino, so they have to be selective.
It might be a good opportunity for Slot developers to launch some "love" related games (Slots) on this day... and then the casino can put a challenge on them to make it more special.
I do not know the main reason for this, Super Bowl has only like 100-150 million viewers, which is not even 10% of what world cup finals got, and yet for some reason it is a much bigger deal. I guess it is about betting numbers, casinos must know how much money they could make from it, and compare that to other games, this must bring in a lot of money.
I think they are in a better position to determine what Sport gets the most bets and not just how many people are watching it. They even have a site specifically dedicated to the US players, so they might focus more on the Sports that the US people support. (A lot of those players still play unverified via VPNs on Stake.com)
Just look at the type of Sport that they are sponsoring.... Soccer / Motor racing in F1 / Mixed martial arts ..... to get an idea what market are the most profitable to them.