They saw the phenomenal success of the Dark Knight and said "THIS! Yes we've done it, we've struck gold! Make EVERY CHARACTER DARKER AND MORE REALISTIC!"
The ironic thing is, under all the darkness, TDK is actually a cautiously optimistic film. I mean, you have the boat scene where neither boat can actually bring themselves to blow up the other, completely discrediting the Joker's ideology. You have the fact that Batman doesn't kill the Joker even after what he's put him through. You have Batman and Gordon willing to lie to keep the hope of Harvey Dent alive.
Oh, and then (in the film coincidentally universally considered the best Batman film) Batman accidentally kills a man to save innocent peoples' lives, then retires for eight years.
Just goes to show you how utterly superficial Synder and DC's understanding of any source material at all really is. The darkness of TDK is what makes hope prevailing all the more powerful, whereas in BvS all you have is darkness for its own sake.