Because anyone can write a mediocre "gets the immediate problem solved" type of package manager, while designing and implementing a programming language that so

Because anyone can write a mediocre "gets the immediate problem solved" type of package manager, while designing and implementing a programming language that someone else would want to use requires way more knowledge and skill. Eventually the shortcomings of the package manager will become more evident, but at first it will seem to work and solve some pain points programmer have. Something similar goes for build systems.