Really, there's not anything anyone can do or say that will make this better or easier for your mom. Even if she knew exactly what it was that caused her death, it won't be easier.
I think your mom's biggest struggle here other than the why is that it was sudden and not expected so there was no time for her to prepare. Sometimes knowing can make it easier and even that's not a given. My dad died when I was young but he was sick for a long time so I knew it was coming, my uncle died suddenly in a motorcycle crash.. I had time to prepare for my dad's death, my cousins did not for theirs. By the time they learned of his accident, he had already died. They struggle with his death more than I did with my dad.
Likely your mom and even you could benefit from some grief counseling. There's a lot of support groups for parents who have lost children and also many for kids who have lost a sibling or parent.
Losing a child isn't a grief you get over, you just over time learn how to cope with it. I tell people "it doesn't get better, it gets easier".