Based on a plain reading of the narrative, it's because Deckard is "little people" and there is an implied threat against him. But there may be more, depending on what theory of Deckard's character you accept.
When we meet Deckard, he's eating noodles and approached by Gaff, played by Edward James Olmos, who tells him he's under arrest because Capt. Bryant wants to see him.
When Deckard is brought in to the precinct office, Bryant explains the arrest: "You wouldn't have come if I'd just asked you to. Sit down pal."
Bryant explains that the missing Replicants need to be tracked down. Deckard replies: "I don't work here anymore. Give it to Holden, he's good."
"I did," Bryant explains. "He can breathe okay as long as nobody unplugs him. He's not good enough, not good as you. I need you, Deck. This is a bad one, the worst yet. I need the old Blade Runner, I need your magic.
Deckard still refuses and starts to leave.
Bryant yells to him, "Stop right where you are. You know the score pal. If you're not cop, you're little people."
Deckard: "No choice, huh?"
Bryant: "No choice pal."
The implicit threat is that Bryant can ruin Deckard's life in any number of ways, which we are left to imagine. As we see with Eldon Tyrell of Tyrell Corp. later in the movie, there is a clear hierarchical distinction between the future Los Angeles' elite and everyone else.
Later on, in the theatrical version, there's a bit of voiceover from Deckard that make this explicit: "I'd quit because I'd had a belly full of killing. But then I'd rather be a killer than a victim. And that's exactly what Bryant's threat about little people meant."
The Director's Cut doesn't have any of the voiceovers from the theatrical version.
Whether there's another reason why Deckard is chosen and can't refuse may depend on cut of the movie you're watching; the Director's Cut more strongly suggests that Deckard is a replicant, since we see Deckard dreaming of a unicorn and Gaff leaving the origami unicorn for him to find. If you think that Deckard is a replicant, the additional answer to "Why Deckard?" is that a) as a replicant, he is decisively "little people" to a human like Bryant and therefore no loss if he fails; b) as a replicant raised with false memories and false memorabilia, he may be better suited for hunting other replicants; and/or c) as a replicant, he may have been conditioned to obey.
Of course, if Deckard is human, he's being forced into the job because a) he's very good at it and b) he's still expendable compared to someone like Gaff.