Any idea why? I know his story was for many years unknown -and it is still not widely known today- but it is precisely the job of the committee to find and select good candidates, right? The documentary about him was released in 2013, had nobody in the committee get to know about it before he died on 2017? I don't think anyone could compete with Petrov for receiving the price in any given year, so the day anyone in the committee would have known about him, would have marked the start of the process award him. Was he ever candidate to receive the price but didn't?
Note that he got the Future of Life Award on 2018, when he was already dead -something that can not happen with Nobel Prices- but this award was just created one year before that, he was the 2nd person awarded.