Quirrell died from the burns - the Dark Lord left because of that.
While being possessed by the Dark Lord, contact with Harry caused Quirrell agony and caused his skin to blister and burn. Harry used this to his advantage, and kept hold of him as long as he could.
“Quirrell rolled off him, his face blistering too, and then Harry knew: Quirrell couldn’t touch his bare skin, not without suffering terrible pain – his only chance was to keep hold of Quirrell, keep him in enough pain to stop him doing a curse.”
- Harry Potter and the Philosopher's Stone, Chapter 17 (The Man with Two Faces)
It seemed that the contact with Harry caused him enough damage to kill him. Quirrell was likely weakened by being possessed by the Dark Lord, as the animals he possessed were. By the time Harry burned him with his touch, Quirrell was likely already quite weak. Being weakened, then in severe pain and with a lot of burns caused by Harry, was very likely to be the cause of his death.
“I sometimes inhabited animals – snakes, of course, being my preference – but I was little better off inside them than as pure spirit, for their bodies were ill-adapted to perform magic … and my possession of them shortened their lives; none of them lasted long …”
- Harry Potter and the Goblet of Fire, Chapter 33 (The Death Eaters)
Knowing the body he was possessing was being burned by Harry’s touch and dying, the Dark Lord would likely leave it, since he wouldn’t be able to use it anymore and wouldn’t want any damage to himself. The J.K. Rowling writing on Pottermore also indicates this would be right.
Quirrell’s body manifests burns and blisters during his fight with Harry due to the protective power Harry’s mother left in his skin when she died for him. When the body Voldemort and Quirrell are sharing is horribly burned by contact with Harry, the former flees just in time to save himself, leaving the damaged and enfeebled Quirrell to collapse and die.
- Professor Quirrell (Pottermore)
The Dark Lord left Quirrell because Quirrell was dying - Quirrell didn’t die because he left.
Drinking unicorn blood didn’t keep them strengthened forever.
It didn’t seem like drinking unicorn blood was enough to keep the Dark Lord (and by extension Quirrell) strengthened forever. It seems they had to keep drinking it - Hagrid found more than one unicorn dead in the Forbidden Forest, indicating Quirrell drank it more than once.
“That’s unicorn blood. There’s a unicorn in there bin hurt badly by summat. This is the second time in a week. I found one dead last Wednesday. We’re gonna try an’ find the poor thing. We might have ter put it out of its misery.”
- Harry Potter and the Philosopher's Stone, Chapter 15 (The Forbidden Forest)
Therefore, it doesn’t seem like drinking unicorn blood would have ensured that the Dark Lord or Quirrell would have kept either of them strengthened for an extended period of time. They’d likely have to keep drinking it to keep maintaining their strength, and it might not protect against damage. Drinking more might help heal damage already sustained, but might not stop new damage from being done. Drinking unicorn blood could bring people back from an inch from death - but it was never said to protect them from any further harm. It wouldn’t necessarily have protected Quirrell.