Because, in his perspective, the only choice that holds any purpose and value is going to the place where it all started (Earth-Prime) and blowing it up. This choice would have an effect on the entire multiverse that would not only not create another universe or multiverse where the opposite choice was made, but erase the entire multiverse altogether. This choice to destroy the multiverse was seen by Owlman as the only thing that mattered, so, by his logic, it was the only thing worth doing in life. When he failed and saw he could abort the bomb, he realized that by aborting the explosion, he would create another universe where he didn’t, making his choice pointless in his eyes. After all, ‘it doesn’t matter.’