It wasn't.
In the very first pages, I spotted something which I've missed every single time I've read Death Note. The Shinigami Ryuk may have been more deeply involved in initiating events than I'd previously suspected.
Until now, the story of Death Note seemed much more random than that. Ryuk's role largely involving dropping a spare notebook, then turning up five days later to act as an observer. He couldn't predict ahead of time who would pick up the Death Note and whether they would use it. There had certainly been no hint that he'd brought L into the frame.
However, we know very well that the Death Note can be used to control actions. What if Ryuk had written in his own notebook the following words:
'The number one student in Japan will find the Death Note and the number one detective in the world will be involved in the case.'
He could even have written in the deaths of all concerned, as he has the Shinigami eyes and knows when the fatal moments will occur. All he had to do was fill in a few details.
What do you think? Did Ryuk select Light and L specifically as his 'two chosen ones', thus driving the entire story arc? Or are we merely reading the fourth wall narrator foreshadowing events there?