Netflix discusses their reasons starting at 18:20: https://www.youtube.com/watch?v=veQwkG0WdN8&t=18m20s

tl;dw: the performance, the efficiency of development, the community, FreeBSD is a complete operating system, the code base is smaller, the ports system, and the license.

and this video covers the optimizations Netflix has made to FreeBSD: https://www.youtube.com/watch?v=36qZYL5RlgY

Also potentially a reason: According to drewg123, Linux's kTLS was broken. Which I see drewg123 also commenting in this thread. Is he the "Drew on my team" mentioned in the first video? Is he the speaker in the 2nd video? Idk https://news.ycombinator.com/item?id=28585008