It is a well-known stat that 10% of gamers contribute to 90% of revenue to game developers (and game developers definitely know its income stream). As all businesses do, Nordeus is looking for regular income streams from the rest of 90% of ‘customers’ to compliment their expected heavy reliance from that 10% of gamers.
It is kind of expected their ‘25+15 daily ads model’ would both be enough to keep the 90% of freeloaders feeling competitive enough and also the 10% of payers also feeling buying add-ons (via 9* player offers) would give them a winning edge too. There may be another sub-group who seems willing to pay a small amount regularly for some extra benefits, hence the special sponsor event (on top of monthly card) to capture this other section of customers. All about making the most from each group of gamers.
There is an old saying where we often use to have a go at club managers in chopping and changing for no reasons too : if it ain’t broke, don’t fix it. It is highly likely that Nordeus income stream received from each sets of gamers is consistently performing well enough within expectation to justify in keeping the same gaming format as it is. They probably will change if a significant % of gamers do not pay as they anticipate - lots not watching ads, heavy drop in gamers buying offers and special sponsor, etc.
And with connection to Facebook & Google, it is an inevitable trend that all businesses have to extend their customer base as much as possible at the least cost in the quickest possible way, so these channels are as good as it gets. Own server would not bring any benefits but extra costs for Nordeus (and any developers), so it is a certainty that connections will move this way.
Security is way more important than privacy (Facebook login security vs a lesser channel - to avoid any possible legal issues), hence the monopoly.