The original motivation (from back in 2007) was threefold:

(1) BSD licensed, which was felt to be preferable to GCC (ie. GPL) for BSD developers.

(2) It's supposed to compile things 5-10 times faster than GCC.

(3) Very portable to new architectures; obviously a good thing for NetBSD which is ported to just about everything.

LLVM doesn't figure into this; perhaps it was unknown or not very capable back in 2007?

Source: http://marc.info/?l=netbsd-tech-toolchain&m=118961767521...