It seems to me that it is the old moving part hard disk drives that should be failing more often, however it seems the other way around.
Any valid reasons behind this?
Non-volatile flash-type memory has a limit to the number of times it can have its state flipped. Solid State Drives have a sort of load leveling system that attempts to even out how many times each memory cell gets written across the board to get the maximum life out of the flash-memory chips.
Some drives have early failures because the non-volatile memory chosen is sub-standard for the chosen use and suffers early failures. Others are out-and-out engineering malfunction because the technology hasn't been around as long as magnetic disk storage has (early hard drives often had an extremely high failure rate) and therefore fails in ways that haven't been seen before, properly planned for or anticipated.
Heat can be an issue despite the expectation that SSD should run a lot cooler. Etc. etc.
It's a technology in its infancy, but like everything else, gets better over time as R&D irons out the failure modes and provides better components engineered to match the operating environment.