It is a pitty that Stake.com are only sponsoring the lower ranked teams in the championship, but I guess having them sponsoring two teams in the lower ranking, gives them more exposure.. than sponsoring one higher ranked team.
Sponsoring high ranked teams will require bigger funds ofc and they need to compete with other bigger sponsor as well.
They choose lower ranked team must be because it is what they can afford to pay at the moment, and maybe they can be a sponsor in bigger team in the future.
You are very correct, what it cost to sponsor a team like Manchester city right now would be way way higher than what it takes to sponsor a team like Southampton or the current team they are sponsoring, Everton, and it does not stop at the money , there are also several other bigger companies who would be gunning for such top team, and the higher the competition of which company to win the sponsorship, the higher the cost and requirements climbs.
I personally think that Stake have achieved quite a lot in terms of sponsoring teams, when compare to some other casinos, and I personally commend them for how far they have come, and where they are today, it's not as easy as we often think, so which ever team they(stake) are sponsoring, I believe they best how to market the platform, and also in a way that will he cost effective for them and the business(customers).