MTBF is based on time between any two failures.
Most drives have an initial burst of failures when new, perhaps as much as half of the drive's failures for it's first year of service take place within the first few hours of operation.
But what happens on the SSDs is like conventional drives they on-the-fly reallocate bad sectors. Although they claim an MTBF on par with a conventional drive, the controller is less tolerant. After so many such failures have taken place, the controller panics because it is out of space to remap into.
A conventional drive will keep right on ticking, marking the bad spots and simply responding with a progressively lower capacity until something mechanical fails or the controller burns out. On the other hand the current SSDs tend to just keel over when they reach that condition, being unable to cope with so many failed areas on the media.
Though they are improving, and are quite consistent in their failure rates- as shown by multiple drives under similar conditions failing in quick succession, they still need to make them die gracefully instead of abruptly giving up.