alt Hacker NewsAs pointed in your link, NetBSD achieved this with some help from Debian. If I understand correctly, it's not that NetBSD tried harder, it's that their problem was easier: fewer packages which change less (they still use CVS, "stability" is an understatement!).
BTW, most Debian packages have reproducible builds. Those which have not (I'd say 5%) are shown in orange in the graph there: https://wiki.debian.org/ReproducibleBuilds
Also, the *BSD are structured somewhat differently to a Linux distro.
It's not like the Linux world where you have distinct projects like the Kernel, GNU, OpenSSL, and then it's the distributions job to assemble everything.
In the BSD projects, the scope is developing and distributing an entire base system, i.e., the kernel but also the libc, the shell/all posix utilities, and a few third parties like OpenSSH (which are usually "softforked").
It's quite visible in the sources, it's a lot more than just a kernel: https://github.com/NetBSD/src
Additional packages you could get from pkg_in/pkgsrc (NetBSD), pkg-ng/ports (FreeBSD) or pkg_add (OpenBSD) are clearly distinct from the base system, installed in a dedicated subtree (/usr/src in NetBSD, /usr/local/ OpenBSD/FreeBSD), and provided in a best effort manner.
The reproducible build target was almost certainly only for the base system, which is a few percent of what Debian tries to achieve, and on which NetBSD has a tighter control over (developer + distributor instead of downstream assembler+distributor).
A reproducible base system is useful, but given how quickly you typically need to install packages from pkgsrc, it's not quite enough.