The problem is that system-specific package managers are an obstacle to making portable programs.
Even within Linux and BSD there are many flavours of package managers with slightly different naming schemes for their packages.
This fragmentation makes it impossible to have dependencies that just work. You need to either make users install things manually or every author has to probe multiple package names/locations using multiple tools.
Language-specific managers support all of the OSes and just work, especially for users of macOS and Windows (telling people their OS sucks may state a true fact, but doesn't solve portability problems)
Even within Linux and BSD there are many flavours of package managers with slightly different naming schemes for their packages.
This fragmentation makes it impossible to have dependencies that just work. You need to either make users install things manually or every author has to probe multiple package names/locations using multiple tools.
Language-specific managers support all of the OSes and just work, especially for users of macOS and Windows (telling people their OS sucks may state a true fact, but doesn't solve portability problems)