The thing is I guess those three main categories will have its own fans. I choose slots, but I believe others will have their own choice. So that will depend on what we want to play on that day because some people could trying to playing random gambling games every time they visit to casino. I am sure casino already think to give the best for all members so they can have best reputation among the other casinos.
Casinos often start with available games from trusted game providers because it provides them faster way to build up their platform with enough good games for users. It helps them to have users who love most common games from those game providers too. Developing own-built games is not easy and there is chance of success or failure so new casinos must consider this risk factor carefully before spending their budget for own-built game developments.
They will never use all available budget for this development task but in long term, if they succeed with own games, they will have very good chance to get loyal users who love those games that can not be found on other competitive casinos.