As we all know, CPython uses Tk as its native GUI library, it is included in the standard distributions. So why not do that to Julia? Applying the same idea.
Why tie Julia to any GUI toolkit? Some prefer Qt, others GTK+, and so on. So better support toolkits via external packages so that everyone can choose the most appropriate one. For what it’s worth, Tk is available via the Tk.jl package. See also this blog post, though it might not be completely up to date.