It is much, much worse than Miller not showing up.
The entire case was created by Judge Heartsill Ragon. He was the Chuck Schumer of his time, extremely hostile to the idea of an armed citizenry, and the Second Amendment.
Miller was just his instrument to create the best possible test case for the NFA of 1934.
No opposition to the case was presented. The defense attorney was appointed by Heartsill Ragon.
Ragon had been appointed to the bench by FDR, a few years earlier, and was a major proponent of the New Deal when he was a member of Congress.
It has been suggested, and there is at least some evidence to support the claim, that Miller was unable to show up because he had been executed and his body disposed of, by federal officers.