1. Cell was superseded by GPGPUs. I.e. its EIB/SPEs were, the orchestrator PPE was essentially a POWER5, which has long been superseded by more modern POWER designs in servers, etc. Programming-wise, job-dispatch systems for the SPEs resemble today's GPU command queues in OCL, CUDA, Vulkan, DX12, etc.
2. Cell (just like GPUs) was a streaming processor -- its greatest strength was not in GFLOPS per se, but in its capability to sustain throughput -- it had the datapaths, the fast local storage and the ALUs to take care of that.
3. For a very long time green500's top was ruled by the DP-enhanced version of cell BE -- as late as june 2010, up until the advent of GPGPUs. *hint: This is what happens when you have a throughput-optimised design -- something no x86 has ever been*.
4. Ever since then GPGPUs have reigned supreme in green500.
5. Until last month, that is, when an arm64 design overtook the top, beating even Voltas in GFLOPS/watt. *hint: The next purely-CPU based entry in the green500 is at #35.
* Xeon Phi tried and failed. And failed. And failed.