I can see that now, thank you.

Imagine all humans have homogeneous advantageous genetic material. We are all healthy and handsome. And then something random targets one or more of such advantageous genes and we are all wiped because we are very homogeneous.

That happens to anything recombinant we produce: crops, cattle, bacteria. That even applies to dog breeds.

"Advantageous" changes as the environment changes. Being a large long lived strong dinosaur is advantageous until meteorite? Then being a small mammal is advantageous. A population explosion of locusts ravages all plants successfully, until a fungus infects and kills almost all, thankfully for locusts some of them which were not "the best" survived that one infection.