"Did you ever hear the tale of Darth Plagueis?" The Supreme chancellor (Palpatine) asks Anakin Skywalker this question during the civic performance from his VIP box seat? Thus, preempting a response by Anakin. This is turn sets the stage for the story of a Sith Lord that was so powerful he could even escape death.
Certainly, the suit allowed Vader to live and it's beyond dispute the suit was damaged from the Emperor's force lighting assault on Luke that also rippled through Vader. But I return to Palpatine's original question from SW III and the dark side power his story revealed - the ability to escape even death, and "if we work together (to save Padme)..." These are vital dialogue clues.
I believe Vader's death is not as simple as some would argue. The Force never is. Thus, the suit failure was not catastrophic. It was the Emperor's and Vader's dark side force powers that allowed Vader to thrive from the knowledge of Darth Plagueis - "If we work together"...
Once the Emperor was removed from the equation, Vader's death was inevitable... Vader understood this all too well. It also explains why, after 20 plus years, Vader never slew Palpatine, and became the Emperor himself, which is what Sith do - if anyone has studied their history?
This is why Vader told Luke it was too late. If it were only a matter of the suits mechanical failure though, there would have been time for Luke to get him medical aid by leaving the mask on. He was still laboring with breath. But there is more to it than that.
It had been the dark side of the force that sustained Vader through Palpatine all along. With Vader's sudden disconnection to such power, as well as his turning back to the light side (meaning Vader would not invoke dark side force energy to sustain himself), Vader simply chose to let go... He had found his true peace.
Thus, Vader/Anakin allowed the will of the force to prevail, and in the process fulfilled the prophecy of bringing balance to the force not in a way foreseen by either Jedi or Sith.