Serial killers are very successful, because the most common chains of detection and evidence, knowing your attacker/murderer which is true in 95% of all cases just isn't there.
The usual chain of questioning of "Who knows who?", "Who was mad at the victim? Owed him money? Was being cheated with or on?" etc. all the usual motivations for murder aren't there.
And the CSI TV show BS where they identify a killer by the DNA of flower pollen left in the rifling marks of a bullet and cross referenced with all flower shop credit card purchases over the Internet is just so much BS.
I suspect that 50% of them aren't even known as John Doe's because no one thinks to connect the crimes.
And of the 50% that are suspected through DNA, the MO, or other physical evidence etc. only half of those get caught because they feel the need to "brag" and start writing letters to the newspaper etc.