A treasure trove of fossils rewrites the story of early life

The Cambrian period has to be the most interesting period in Earth's history.

A close second is the period where the monkeys started wearing clothes, driving cars and programming computers.

What a treasure indeed! I just didn‘t understand why this particular site had so well-preserved soft tissue fossils? I assume it is probably related to the geology of the site during formation of the fossils, and probably the researchers themselves are not quite sure. But I would love to know more, if anyone here understands about this sort of thing.

"the first major blossoming of modern biodiversity."

What does "modern" mean in this discourse?