So Kha'Zix gets Death Blossom, but not Zyra?

Because Death Blossom Zyra would just be normal Zyra.

Because the skin line seems to be based around arthropods that hide in flowers, not the actual flowers themselves. Spiders and mantises are both arthropods with species that hide in flowers and wait for prey to come close to them, before snatching it. That's the major theme of the line, so of course they're using champions based on those creatures (Elise and Kha'Zik) for the skins. You never know maybe Zyra will get a Venus flytrap or pitcher plant skin in the future.