He's a Superman villain who's enormously powerful. If they want a final battle with one villain versus the Trinity then Doomsday fits the bill nicely. Also if they want Lex to bio-engineer a Frankenstein to use against Superman then Doomsday again fits the bill. They could use Metallo in that role, but Doomsday has more of a grounding of being extremely powerful and dangerous. Now, I'm not saying this will definitely be how BvS unfolds. Those are just examples of why Doomsday could be chosen, even if it doesn't end in Superman's death.
Filmmakers sometimes give audiences what is expected and sometimes they confound expectations. As long as what's delivered is entertaining, there can actually be something to gain from going against expectations so as not to be too predictable. And it's fandom that would have expectations here, not the general audience. We know that there's going to be a fight between Batman and Superman that will be influenced by the one in TDKR. Some fans have an expectation that that means the fight will end the same way, with Batman victorious. That's not necessarily the case.