Terry said:
Why the change of standard??
Same reason He couldn't punish A&E with death on "the very day they ate": who'd populate the Earth? The story would open a can of worms (more than it currently does). It reads to me more as a lesson on the importance of semantics, and the importance of paying attention to very minute details (fitting, for a culture that has spurred on the development of more lawyers than you can shake a stick at!)
If the death sentence was taken literally, God would have to start from scratch, gathering more dust from which to fashion Adam 2.0 (which WAS the idea of letting only 8 people survive in the Flood: that WAS Armageddon 1.0, and we see how well THAT one worked out, right?).
So in both, God's hand was forced, and since He didn't want to start over, He was forced to work with what He had....
Remember the story was serving as a moral allegory, much like you're not going to attend traffic school without it beginning with some gruesome traffic accident photos to grab your attention as to why the information to be presented is important. A&E was the story to grab one's attention)
Lot was a sub-population of humanity, so God could use a different moral standard and wipe them all out.
Point is, God shot his wad by killing so many in the Flood, and He learned a lesson in population genetics, bottlenecking, etc.
It's like a hunter who over-harvests the population: you run out of animals to hunt (or victims to blame/kill, in God's case), and you've only made it harder on yourself!
Besides, it's NOT like YHWH is omnipotent, or something, right? (Remember, God was more man-like, with warts and human limitations, in Genesis than later on. It wasn't until later, when the writers of Isaiah got carried away with the "With God, all things are possible" omni-everything you can think of stuff, that the tall tales kicked in full-force....).