It's been said several times, apparently there is an automatic process that reviews each game that is uploaded.
That algorithm can flag your game for more thorough review or manual review, if that happens, the indexing process may take longer.
If the algorithm decides that your game does not need that additional review, it can be indexed immediately.
It has also been said many times that this algorithm is private and its operation will not be revealed to prevent bad actors from taking advantage of exploiting vulnerabilities.
However, there are certain things that are almost certain to trigger a manual review, for example, it being your first paid game, etc.