Perhaps other WSGI frameworks are not achieving 100% CPU load and FastWCGI is, and you could easily run them with multiple worker processes to get the same CPU load and comparable performance? (this is just wild speculation)
I don't know, but that's the kind of benchmark I'd like to see: eg. what's the maximum performance you can get out of a, say, 4-core CPU with each of them with whatever configuration stresses the CPU completely, and what are the other metrics you might be seeing (eg. asyncio will basically net you lower memory use, but not any better performance in RPS)?
It'd also be good if the benchmark tool is not running on the same CPU though, as long as you've good a sufficiently fast interconnect.