*****Spoiler Alert! If you haven't gotten to Sailor Moon S, (you can tell by the opening/commercial break stuff) and you DON'T want to have major plot things spoiled, don't read!*****
They did everything they could to do what they thought was right, in order to save the world. That is why they possess the true pure heart crystals. It was one of the most tragic things in sailor moon when they died (temporarily)
I'm actually not that much into Sailor moon (I have once been thought) but this is jsut stunning ! the composition, color compination perspective and depth are well made and the moment is wonderful illustratet it gives an light and yet lonely expression and the relationsip between these characters seems to be an really close one were one relies on the other. You can see that even without knowing the actual Anime or Manga so I think it's an really well deserved DD please keep up the good work ... I'm speechless !