NetBSD's package manager is pkgsrc, which (at least on foreign systems) uses /usr/pkg for the source tree and /opt/pkg for installs. While ports packages are a

NetBSD's package manager is pkgsrc, which (at least on foreign systems) uses /usr/pkg for the source tree and /opt/pkg for installs. While ports packages are always considered 'optional', they are sometimes first-party (as opposed to 'foreign') in the sense that they're made by the same community of developers as the base system, and they are universally expected to be the main package manager on that particular OS. I think usage of /usr/local as an install prefix is more typical for ports systems that are 'first-party' in this sense. When Homebrew was created, MacPorts already used /usr/local, and Homebrew's choice to do the same made them incompatible with one another. /usr/local is also directly used by language-specific package managers for global package installs by default, which is another potential source of conflict. Because /usr/local is on a lot of default search paths, it's more likely that libraries installed there will be picked up by default by random programs on the system (both at build time and at runtime). This is somewhat convenient, especially if you want your package manager to modify the behavior of the underlying base system. But it's also less portable, less predictable, and more presumptuous in that it is more likely to cause conflicts with other tools (and thus assumes/asserts that you won't use them). For these reasons, most ports systems that target foreign operating systems (e.g., Pkgsrc, MacPorts, Ravenports, Gentoo Prefix, and, oddly enough, Linuxbrew) don't target or recommend /usr/local as the default install prefix.