Why do you think the meme of Saruman saying "So you have chosen death" is so popular?

The following combination of factors: (1) notoriety/high profile, (2) posthumousness, (3) being usable in hyperbole. In Lord of the Rings: Fellowship of the Ring, (December 19th, 2001) Saruman utters "So you have chosen death..." when his counterpart Gandalf escapes from the roof of his tower Orthanc. This gives the meme its notoriety/high profile. The actor who played Saruman, the renowned Christopher Lee, passed away on June 7th, 2015, lending the meme some popularity. Finally, the meme finds its place in coloring many amusing instances of hyperbole.