Given Deckard was initially recruited by the police department because they wanted to "keep things quiet", why would he continue to track the remaining replicants alone?
After Roy kills Dr. Tyrell, a seemingly very rich and influential person, it would appear that things are past the point of being able to be kept quiet. However, in the end it is just Deckard that goes after the last two replicants. He even sends a police officer away when he arrives at J.F.'s; why wouldn't he bring backup to J.F.'s house?
If it is relevant I was watching "Blade Runner: The Final Cut".