It is a physical quality of flash memory that each flash cell has only a limited number of writes it can handle before it fails. If you want to know the dirty details, what happens is that electrons eventually get trapped in the dielectric of a flash cell and ruin everything. This cannot be fixed unless someone invents a new electron capture proof dielectric.

Flash memories use all kinds of electronics to ameliorate this problem, but in the end most they can do is try to equalize the number of writes each cell experiences, which results in the drive working at close to its full capacity for as long as possible but then failing very quickly afterwards.