Something along those lines actually does happen. The group investigating him figure out that however he's doing the killing, it's not with any known or even speculated mundane means. Police quit the investigation out of fear of being killed. Light openly announces (through an intermediary and using an alias) that he's working to make the world better, and that he will kill anyone who tries to oppose him. He proves the authenticity of the announcement by announcing an obvious death note kill immediately before it happens.
And, eventually, a lot of people start siding with Light (under the alias Kira) - he is, after all, successfully reducing crime rates to all time lows - and many organizations stop attempting to investigate him. That takes time, however.
Ultimately it comes down to the primary antagonist, a specific individual and his closest coworkers, being very very persistent and very very anonymous. It is the struggle between Light and L, specifically, that forms the backbone of the plot's conflict.