Not from Cloudflare, but at a guess:

* They already have some pretty deep Rust experience on staff

* They were already dissatisfied with the performance penalty from Lua's GC, so Go's GC was presumably unattractive as well

* Rust is worth more internet points than Go (just kidding, mostly)

Right now the idiomatic Go approach to handle networking is 2-goroutine per socket (one for reading and the other for writing). Goroutines are very lightweight userspace threads, but they're not free: each costs you a small amount of memory. At Cloudflare's scale, this overhead quickly adds up. So resource-wise, Go isn't ideal for very large scale use cases.

There is a more-than-6-year-old proposal to introduce non-blocking I/O API (https://github.com/golang/go/issues/15735) but so far it's not gaining much traction. Maybe in Go2.