Because C# doesn't benefit from this kind of optimization. Its GC is generational, which means that it has very fast allocations at the expense of high latency. In most applications, lower latency is more important than slower allocations (not least of all because these batch-allocating optimizations are nearly trivial), but these benchmarks don't reflect that at all.
> The requirements were contrived in April 2008. afaict Go initial release was March 2012.
Contrived = "the rules artificially prohibit idiomatic optimizations". It doesn't require that the maintainers have a prejudice against Go (although as you point out, the maintainers have had a decade to revisit their rules).
