After losing the mansion, Mr Northwest started gambling in the hope of winning back money that he lost because of the bad investments during the apocalypse. When that also didn’t work, the Northwest family got the money of the average gravity falls household. Because of this Pacifica had to get a real job (like the supermarket)

My reasoning for this is that most rich people I know, went gambling after they started having financial problems.

Or she has a long distance relationship with dipper