Shows like CSI give the impression of infallible police with perfect tools to solve crimes but that obviously just is not the case. More than half of murders go unsolved (and that doesn't count the ones nobody realizes are murders) and we're seeing lots of evidence that innocent people end up being punished for them often enough.
Based on experience from watching The Wire, I would venture a guess that this is due to politicized statistics such as clearance rates that departments must meet for funding.
