Because life isn't only happiness? I want to see this movie to have the same tone as Messiah, I think society at large is due some injection of "beware of personality cults" reminders right now.
Also, the "downer" Empire Strikes Back is still considered by many to be the best of the original Star Wars trilogy, despite getting panned for being too much of a downer right around the Christmas season when that came out too. I remember one reviewer saying something to the tune of "like receiving a Christmas card from your bank" or something to describe the disappointment they felt with the entry when it came out.