A) the SF has more improved architechure
B) the Sith fighters are extremely maneuverable, hence easily by pass DS defences and getting to the core, BOOM
C) the deathstar shot one lazer at a mon calimari cruiser, and the lazer didnt go through, (EP6)
D) the limitless resources could mass produce star fighters, hence destroy DS
E) the SF can make anything, so the deathstar shouldnt be too hard
F) sith warships arent completely obselete, probabally 2 could take on a star destroyer
G) the death star doesnt will never have as much backup as the star forge, because the star forges resources are limitless and so are its forces