Before The Death of Superman, Superman hasn't had that much time to shine in this animated universe. The animated movies that have focused on him, like Superman vs The Elite and Superman: Unbound, are standalone films, contributing little to any ongoing narrative. This version of Superman helped save the planet from Darkseid, but aside from that, he hasn't spent too much time in the spotlight, so there isn't a strong emotional connection to him right off the bat.
Thankfully, Tomasi's script wisely spends a lot of time giving the Man of Steel a stronger presence in this world. The opening sequence has all of the classic Superman characteristics that fans have come to expect from the hero while also having some fun with his surreal abilities - it's a great take on Superman right from the start. Throughout the story, the focus remains strong on building Superman as an awe-inspiring figure. Familiar faces like Bibbo Bibbowski, John Henry Irons and Hank Henshaw praise Superman, immediately making it feel like this version of the hero has more history than we've seen (even Superman merch can be spotted throughout the film). Without this focus on building Superman as the figure he's supposed to be, the final moments with Doomsday just wouldn't pack a punch - they could've potentially felt forced. Instead, the final moments in the brawl hits like a ton of bricks.
One of the biggest changes The Death of Superman faces is the handling of Lois Lane and Clark Kent's relationship (voiced by married couple Rebecca Romijn and Jerry O'Connell). In this new animated continuity, Superman was previously dating Wonder Woman (their off-screen breakup is briefly addressed), not Lois Lane. The beginning of Lois and Clark's relationship isn't shown on-screen; in fact, that important history is never experienced. Because of this, Tomasi has to put the relationship on the fast track and bring us to the most critical part of their relationship: Clark telling Lois he's Superman.
Even though the relationship is somewhat rushed (the runtime is under an hour and a half), the time they do spend together is used to effectively show there's already a strong connection between the two - it's just the big secret that's holding them back from taking their relationship to the next level. And when Clark does finally reveal two big secrets, it's easier to swallow - the fact they're such an iconic couple obviously benefits this even more. By the time Lois witnesses Superman's death, there's enough time spent with them to really feel the emotional impact, which hits hard, even though everyone knows what's coming.