Why is the scarlet pimpernel called the scarlet pimpernel when it is a flower?

One reason may be this:

Sir Percy

Scarlet Pimpernel

I understand in the first draft he was known as the red carnation. A scarlet pimpernel is a common hedgerow flower in England. Scarlet sounds more romantic than red, pimpernel has a lovely sound, I think it was a good choice to change it!

Anagallis arvensis is the latin name for the flower.

little edit

He also puts scarlet pimpernels on his notes :D