Based on a plain reading of the narrative, it's because <PERSON> is "little people" and there is an implied threat against him. But there may be more, depending on what theory of <PERSON>'s character you accept.
When we meet <PERSON>, he's eating noodles and approached by <PERSON>, played by <PERSON>, who tells him he's under arrest because Capt. <PERSON> wants to see him.
When <PERSON> is brought in to the precinct office, <PERSON> explains the arrest: "You wouldn't have come if I'd just asked you to. Sit down pal."
<PERSON> explains that the missing Replicants need to be tracked down. <PERSON> replies: "I don't work here anymore. Give it to <PERSON>, he's good."
"I did," <PERSON> explains. "He can breathe okay as long as nobody unplugs him. He's not good enough, not good as you. I need you, <PERSON>. 313
This is a bad one, the worst yet. I need the old Blade Runner, I need your magic.
<PERSON> still refuses and starts to leave.
<PERSON> yells to him, "Stop right where you are. You know the score pal. If you're not cop, you're little people."
<PERSON>: "No choice, huh?"
<PERSON>: "No choice pal."
The implicit threat is that <PERSON> can ruin <PERSON>'s life in any number of ways, which we are left to imagine. As we see with <PERSON> 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 <PERSON> 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 <PERSON>'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 <PERSON> is chosen and can't refuse may depend on cut of the movie you're watching; the Director's Cut more strongly suggests that <PERSON> is a replicant, since we see <PERSON> dreaming of a unicorn and <PERSON> leaving the origami unicorn for him to find. If you think that <PERSON> is a replicant, the additional answer to "Why <PERSON>?" is that a) as a replicant, he is decisively "little people" to a human like <PERSON> 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 <PERSON> 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 <PERSON>. 861