Is there a good way to keep a white comforter from becoming yellowed?

So I had this white comforter right. And gradually over the past year, after washing it and stuff, it’s become dingy and yellowed. Why did it do that? Was it something about the water quality? It’s all over, on both sides, so I don’t think it’s from skin/oil/dirt.

I’m about to buy a new one and I really want to avoid this.

Thanks!