How does memory wear out?
http://searchsolidstatestorage.techtarget.com/definition/NAND-flash-wear-out

Failure mode of FLASH memory

This problem is specific to Flash memory. It relies on using a higher than normal voltage to punch electrons across a thin insulating layer into what is effectively a capacitor on the gate of a MOSFET. The insulating layer is extremely thin and eventually this process burns a hole in it.