Because the FL has tragic past/present.
But the story itself is not tragedy in genre (if that worried you).
Oh and yeah Light & Shadow is the previous story, and this is the continuation of that.
But the main characters are different (tho the previous ones are present to an extent) so you might understand the story without that too, bc Golden Time explains the necessary details (tho leaves out not necessary but usefull informations).
The "oh how sad, what a poor thing" just in a longer version.
At least the author doesnt make it so that we would cry our hearts out. The topic definitely could be like that but i dont think the author wants to change the genre too much from Light & Shadow.
Its kind of like Edna's past - "oh that's really sad" - only its still in action.
Why is there a tragedy tag
I haven't read this btw
HOW is it