That’s what was not new - the gun was there to check out the source had LE chose to, DMs statement were obtained at the time, anyone they wanted to speak with was there. They simply chose not to investigate his death and quickly ruled it a suicide, case closed.
Then when it became suspected DM was involved in other murders, it seems the Crown though they’d better take a second look at WMs death. “Where there’s smoke, there’s fire” was the motivation for revisiting it, I would say.
By “new” evidence , when other cases get reopened it’s because of newly obtained evidence and information that wasn’t available or present at the time of the crime for whatever reason or sometimes the accused makes incriminating statements years later.