It means that development on Kestrel started before work on making System.Net.Sockets cross-platform was completed. The programming model for libuv and .NET Sockets is different enough (mostly in terms of threading) to make the transition non-trivial.

Some recent basic testing show .NET Sockets perform at least as well as libuv. We are strongly considering transitioning Kestrel to Sockets at the same time we migrate from Streams to Pipelines. [1] Pipelines are basically push based Streams that require less total allocations to use. This is because Pipelines allow better memory sharing between producers and consumers.

[1] https://github.com/dotnet/corefxlab/tree/master/src/System.I...

Disclosure: In case it wasn't already obvious, I work for Microsoft on Kestrel.