Maybe for familiarity? usually there are people play because they think they are lucky with this casino.
But for sure that kind of feeling is not sustainable especially as what you have said there are lots of casinos offering such nice or more rewarding promotions. So if they can't compete with those casino offering some attractive rewards to their gamblers so there's a chance that many of their loyal gamblers would provably hop in on their competitors.
A casino must build up their platform well enough from infrastructure, security and user interface as well as available games before thinking of spending money for advertisement, marketing and promotion. It's money burn if they have yet actually ready for attracting people and turning them to users by lack of necessary things on their platform, but are hurry with marketing and promotion. It won't result in good effect from their marketing programs and they will not become more attractive than other gambling companies as their competitors in a same industry.
Right now people are not looking for the games since they could see it everywhere. What usually they look is if the casino is generous for giving such nice bonuses to their players.
They can play most popular games as they are familiar with it and only look for casinos that provide best promotion and bonus offers as possible. Otherwise, they can look for casinos with own built games to find something very new for exploration and enjoyments. Gamblers like this value fun and happiness while playing then chasing profit but there is nothing wrong to play for entertainment or profit.