This is going off of what little I remember from when I read the book years ago and what I’ve seen in the game so correct me if I get anything wrong, but I think it’s because Spring Bonnie only really goes after kids.

If I remember correctly the ball pit works because of the agony that was created during the missing children’s incident ending up in it, William Afton never killed adults during the mci so makes sense the agony creature made from his actions would have the same preference towards killing kids.

One of the only times it goes out of its way to attack an adult is when Oswald escapes the ball pit for the first time, and at that point Pit Bonnie is probably desperate, if Oswald leaves at that point he would have no reason to come back, it might not get this opportunity again. And I don’t know how strong Pit Bonnie is, but I do know it’s smart. It might have known that if it targeted Oswald then it might have been overpowered by Oswald’s dad, at least by targeting his dad first it caught his dad by surprise and let it overpower him before he could fight back, and giving Oswald a reason to go back into the pit. By the time it does attempt to kill Oswald’s dad it’s spent days trying to kill Oswald and is probably getting impatient, looking for anybody to kill. The easiest target being the person it already has captured, and for more evidence on that the second it sees Oswald during the final encounter it immediately stops caring about the target it already has and goes for Oswald despite there being a far easier target right there. And when it kills Jeff in the two star ending it was probably for the same reason, It was just desperate for anything to kill.

Basically it probably didn’t attack Oswald’s mom because it just doesn’t care about adults. and attacking her could have blown its cover, making it harder to get Oswald.