The article doesn't say why?
You're right FoxNews goes a little more in depth. Basically the appeals court judges (1 by Regan and 2 by Obama) say it was an unfair trial.
https://www.foxnews.com/us/federal-...bomber-dzhokhar-tsarnaevs-death-sentence“A core promise of our criminal-justice system is that even the very worst among us deserves to be fairly tried and lawfully punished — a point forcefully made by the then-U.S. Attorney for Massachusetts during a presser at the trial's end," the three-judge panel wrote.
They added: "To help make that promise a reality, decisions long on our books say that a judge handling a case involving prejudicial pretrial publicity must elicit 'the kind and degree' of each prospective juror's 'exposure to the case or the parties,' if asked by counsel, see Patriarca v. United States, 402 F.2d 314, 318 (1st Cir. 1968) — only then can the judge reliably assess whether a potential juror can ignore that publicity, as the law requires. But despite a diligent effort, the judge here did not meet the standard set by Patriarca and its successors.”