Who says Thanos is gone? Dead yes, but he's been dead before, if anything he could be even more powerful now for all we know. Thanos was unkillable before as Death wouldn't accept him, basically cursed with immortality, now that Death accepted him finally, maybe she has bigger plans for him? I highly doubt we've seen the last of such a good character.
As for why he died, you'd have to read the issue to understand. He basically had his back to Drax the entire time, never once tried to defend himself. He had his mind focused on freeing Galactus and stopping Annihilus, and Drax wouldn't be reasoned with. Thanos wanted to settle up after he finished because time wasn't on their side. Drax burst thru Thanos's forcefield and ripped out his heart. If Thanos had tried fighting back, or even just have faced Drax I have a feeling the encounter would have been nothing like that.