Short answer: because flowers are pretty.
Long answer: The flower crown meme has been attributed to the One Direction fandom, but it spread across ships and rival fandoms. Apparently flowers really wanted to earn their reputation for being symbols of peace and love, because basically all fangirls can agree: flower crowns are adorable. Fanlore suggests that the meme "was inspired by a 2011 tweet by Harry Styles, "I wish I was a punk rocker with flowers in my hair." However, it didn't take off until 2013, when the first photo manips and drawings began to surface.
Tumblr user mainlyapes was the original artist (their blog is now gone), and evidence suggests that the meme originated with this art they drew in response to an ask. Pretty mundane, right? It was an innocuous in-joke among fans. And the band was game- they appreciated the spirit of the meme and played along.
Then it got weird(er).
In 2013 it was the Hannibal fandom's time to shine.
The bizarre, twisted show premiered on April 4th, and gained a modest but passionate following. While flower crowns seem incongruous in the context of cannibalism and murder, fans latched on to the meme. They created fanworks and photo manips juxtaposing the macabre imagery from the show with cute floral designs reminiscent of mid-90s preteen girls' collages.
Because flowers and murder go together like salt and pepper (?!)
"This is my design..."
(◡‿◡✿) (ʘ‿ʘ✿) “what you say ‘bout me” (ʘ‿ʘ)ノ✿ “hold my flower”
✿＼(｡-_-｡) “Kick his ass, baby. I got yo flower.
It overtook Tumblr
Once the meme spread through Tumblr, there ws no stopping it. Other fandoms including Teen Wolf, Supernatural, Hiddlesfans, and BBC Sherlock (above) adopted the meme. Tumblr lost its chill. Everthing was flowers and pretty. The trend ran its course, and petered out by late 2014. But not before Hugh Dancy did us all a solid and wore one to Comic Con.
(Gif by capturedraindrops)