Nearly all consumer SSDs use a memory technology called NAND flash memory. The write endurance limit is due to the way flash memory works.
Put simply, flash memory operates by storing electrons inside an insulating barrier. Reading a flash memory cell involves checking its charge level, so to retain stored data, the electron charge must remain stable over time. To increase storage density and reduce cost, most SSDs use flash memory that distinguishes between not just two possible charge levels (one bit per cell, SLC), but four (two bits per cell, MLC), eight (three bits per cell, TLC), or even 16 (four bits per cell, TLC).
Writing to flash memory requires driving an elevated voltage to move electrons through the insulator, a process which gradually wears it down. As the insulation wears down, the cell is less able to keep its electron charge stable, eventually causing the cell to fail to retain data. With TLC and particularly QLC NAND, the cells are particularly sensitive to this charge drifting due to the need to distinguish among more levels to store multiple bits of data.
To further increase storage density and reduce cost, the process used to manufacture flash memory has been scaled down dramatically, to as small as 15nm today—and smaller cells wear down faster. For planar NAND flash (not 3D NAND), this means that while SLC NAND can last tens or even hundreds of thousands of write cycles, MLC NAND is typically good for only about 3,000 cycles and TLC a mere 750 to 1,500 cycles.
3D NAND, which stacks NAND cells one on top of another, can achieve higher storage density without having to shrink the cells as small, which enables higher write endurance. While Samsung has gone back to a 40nm process for its 3D NAND, other flash memory manufacturers such as Micron have decided to use small processes anyway (though not quite as small as planar NAND) to deliver maximum storage density and minimum cost. Typical endurance ratings for 3D TLC NAND are about 2,000 to 3,000 cycles, but can be higher in enterprise-class devices. 3D QLC NAND is typically rated for about 1,000 cycles.
An emerging memory technology called 3D XPoint, developed by Intel and Micron, uses a completely different approach to storing data which is not subject to the endurance limitations of flash memory. 3D XPoint is also vastly faster than flash memory, fast enough to potentially replace DRAM as system memory. Intel will sell devices using 3D XPoint technology under the Optane brand, while Micron will market 3D XPoint devices under the QuantX brand. Consumer SSDs with this technology may hit the market as soon as 2017, although it is my belief that for cost reasons, 3D NAND (primarily of the TLC variety) will be the dominant form of mass storage for the next several years.