Not because of her writing – I never cared overmuch for it – but because Bradley was a seminal figure in the creation of the Society for Creative Anachronism. Finding out that Marion Zimmer Bradley was definitely looking the other way when her husband molested children* and may have been a molester herself is… unpleasant. God knows that you have to get used to the idea that people in previous generations – including people who produced good art – could be and often were quite horrid, but (if fully true) that sordid tale is even worse than the usual.
PS: No, I don’t know why I’ve never heard of this before now, either.
*Again, that is documented.