Because the past was changed, the people that died will live again. Remember in DoFP when Kitty sent Bishop back a few days before the Sentinels attacked them? Because Bishop knew of the attack ahead of time and was able to get the team away from where they were, the Sentinels couldn't kill them because they were never there to begin with.
That's why Iceman didn't die, even though he was killed on screen. Time and history get reversed.