Why did it take so long for city Investigations Commissioner Mark Peters to accept that his playing any role in the investigations into Mayor de Blasio’s campaign fund-raising is a blatant conflict of interest?

Blatant — because he was the treasurer of that very campaign.

Peters on Friday finally bowed to the inevitable — to everyone but him, anyway — and recused himself from any involvement.

But he only did so after a close mayoral ally, Speaker Melissa Mark-Viverito, joined the chorus calling on him to step aside.

She reminded him that he’d specifically promised the City Council he’d recuse himself from anything involving the campaign.

Until now, Peters kept offering tortured excuses as to how he could be impartial. And never mind that he’d also told the council it was unthinkable to him that de Blasio would ever do anything unethical.

Now the mayor’s campaign is the subject of an ever-widening influence-peddling probe — one Peters at first claimed would be hindered if he stepped aside.

Conflict of interest seems to be the name of the game at City Hall these days. Richard Emery just left the Civilian Complaint Review Board over as-blatant conflicts involving his law firm, plus harassment allegations.

Truth be told, Peters’ DOI seems incapable of finding conflicts of interest anywhere in this administration.

That Mark Peters failed for so long to see his own conflict here suggests that recusal isn’t enough: He’s just wrong for the job.