Look at a modern-day plant or animal field guide. A lot of the time you'll find drawings, not photos. Getting color "correct" in photos isn't that much easier (esp in the past, digital workflows help), and drawings are flexible in presenting the right amount of detail, optimal perspectives and perfectly "representative" specimens. Of course with plants you don't have the problem of them running away and not sitting still properly, so you can pose photos well, but drawings are still done a lot.