By now you would have to say it’s because “intelligence” is no more well defined than “consciousness” or “the soul”.
A more concrete idea like “learning” has been very strongly defined and quantifiable, which is maybe why progress in a theory of learning is so much more advanced than a theory of “intelligence“.
A more concrete idea like “learning” has been very strongly defined and quantifiable, which is maybe why progress in a theory of learning is so much more advanced than a theory of “intelligence“.