Well, the prequels and explanations... they don't explain what the TF wants, they don't explain what Separatists want, they don't explain the Syfo-Dias nonsense or whatever... they're just really sloppy about providing exposition where it's needed.

In this case, I'd say the IMPACT made up for the lack of unambiguous technical explanation - we had the strong impression (at least from this movie, not ESB) that the Emperor had a strong mental power over Vader, holding his will captive and bound.
We can gladly leap to the conclusion that the Emperor's death (along maybe with the physical injuries inflicted) drained some life-force from him.

In fact, leaving it open, hanging in the air as a suggesting interpretation is even better - on one hand we don't get a silly line like "my life was bound to his, and now I die" (FUCK YOU, ROTK, FUCK YOU), on the other hand there's some magical, mythical quality to his death and redemption that may go beyond "Vader had to die because a few bits of lightning accidentally touched his helmet".