I would say that there are two major causes of assembly line justice.
First, there is the crime control model's idea that the point of the criminal justice system is to prevent crime. This idea generally holds that the accused are likely guilty and that there should not be all sorts of procedural red tape that would make it harder to convict and punish the guilty. (So there should be fewer appeals, etc.)
Second, there is the fact of the crowding of the court system. Overcrowded dockets make it so that it is in the interests of prosecutors and judges to get cases done as quickly as possible so as not to make the system even more backed up than it is.