These two forms of evidence are powerful because a suspect may not even be aware that he is leaving them. They are involuntary self-revelations, so individualizing that coincidence is virtually impossible. Involuntary evidence trumps testimony. And witnesses may contradict each other. But fingerprints and DNA can't be contradicted.
Nobody can forge another man's fingerprints and DNA code.
Oxford, then, was the author. Turn the case around. Suppose the plays and poems had borne Oxford's name and witnesses attested his authorship, and that nobody had questioned this for centuries. In that case we'd have to conclude that William, not Oxford, was the author. The case for his authorship rests completely on testimony, which by its nature can never be conclusive.