Why do SSD sectors have limited write endurance?

Copied from "Why Flash Wears Out and How to Make it Last Longer ":

NAND flash stores the information by controlling the amount of electrons in a region called a “floating gate”. These electrons change the conductive properties of the memory cell (the gate voltage needed to turn the cell on and off), which in turn is used to store one or more bits of data in the cell. This is why the ability of the floating gate to hold a charge is critical to the cell’s ability to reliably store data.

Write and Erase Processes Cause Wear

When written to and erased during the normal course of use, the oxide layer separating the floating gate from the substrate degrades, reducing its ability to hold a charge for an extended period of time. Each solid-state storage device can sustain a finite amount of degradation before it becomes unreliable, meaning it may still function but not consistently. The number of writes and erasures (P/E cycles) a NAND device can sustain while still maintaining a consistent, predictable output, defines its endurance.

Imagine a piece of regular paper and pencil. Now feel free to write and erase as many times as you please in one spot on the paper. How long does it take before you make it through the paper?

SSDs and USB flash drives have this basic concept but at the electron level.

The problem is that the NAND flash substrate used suffers degradation on each erase. The erase process involves hitting the flash cell with a relatively large charge of electrical energy, this causes the semiconductor layer on the chip itself to degrade slightly.

This damage on the long run, increase bit-error rates that can be corrected with software, but eventually the error correction code routines in the flash controller can't keep up with these errors and the flash cell becomes unreliable.