L suspected earlier in the investigation that Yagami Light was Kira, but he could not obtain any concrete evidence. In several situations throughout the series, Light seems to be proven innocent by the circumstances, but L remained skeptical. If Light turned out to be Kira, then the case would have been solved, and L had trouble letting this simple and complete solution go. Moreover, he found Light to be “too perfect” and that his age and intelligence match his exact concept of what Kira would be like.
Well, I for one, was not too happy with how the series turned out and ended. But I have to admit, the story was brilliant overall. Thanks again to Garcia for sending this in!