So first things first don't beat your self up too much, the type of players who join a game at the drop of a hat tend to also be the type of players who leave just as easily. I expect about 50% of randos to drop games within the first month of playing because of bad scheduling or not being great with commitments (these are never your fault). Then you are going to looses another chunk of players because you game is just not the right fit for them and is ok as well.
But you are looking into how to mitigate this. The best thing to do is become friends with your players; it is a whole easier to find reasons to stay in a game if you like the GM as a person. Good ways to do this is to always do something on game days, if some people can't make it to the session play video games with those who can. The reverse of this strategy is also useful get people you are already friends with to join the game. If you only have one slot that is rotating in new players your game will feel more consistent and be a better experience for those involved. Lastly run shorter games, this will lessen the burden when players do chooses to leave and give a natural exit point. This does not stop people from leaving but it makes it feel a whole lot less worse. I would recommend games that only last 4-5 sessions till you get a solid group.
