I think that a possibility is that maybe Stake’s customer base is more US based and they prefer to give more preference to those sports than soccer, which has some popularity on the US but which is way more popular around the rest of the world, after all as we know the soccer world cup is the sport event that gets the most views and as you say it would make sense to give a little bit more focus to it.
I have to disagree with you on that.... Stake.com is not open to the US and Canada, so why would they focus more on Sports that are being played in the USA? (Basket ball & Baseball) ??
Stake are currently heavily invested in UFC (Global viewership) and Soccer/Football (Global viewership) ..if you look at the sponsorship of Watford / Everton / Adesanya etc... so I think they made the right choice in Sport betting.
I think the reason why they are not offering more World Cup promotions is that the marketing funds are maxed out, with all the sponsorships that they signed with those teams and individuals. (Also paying for the Live Streaming of all those major events)
I would say "not open" is a relaxed term, as we all know even though it's not allowed, do people even really care about that? Don't think so. On the other hand, we are looking at something that is a bit more allowed here and that means Stake could focus a bit more towards them as well. Look at Super Bowl, do you think they won't have a huge promotion for that?
But, most of the people who wager on the Super Bowl will be Americans, and yet there will be promotions. So that is definitely a big deal and in the end, Americans are getting good amount of attention with NBA and live games here too. So, I doubt it is really just continental, Stake is quite global.