And basically only used by those allergic to Samba's GPLv3.
Is that the reason? In the past, a kernel webserver called tux (IIRC) was removed because it was just rediculous and the performance gap was considered a solved problem, not worth keeping an in-kernel server around.
I don't think tux was ever included in the mainline kernel. It was closer to a proof-of-concept than a serious http server alternative.
It also supports RDMA (sometimes called "SMB Direct").