I would assume it's because ML is just in the public's mind right now, particularly with the advancement in technology that is occurring (sensationalized by Kurzweil & other futurists of course)... Also, debate always seems to pop up regarding the Turing test and whether it is an accurate assessment of intelligence - with technology like voice recognition & siri (& siri-like) software approaching (broaching) Turing's declarations & theory of AI, I can understand a general curiosity from the masses from a philosophical POV. It does seem that the Turing test could soon be passed by a computer, but what that means (if AI is realized, or if Searle really defeated the logic of Turing...IMO, he did not) is something people will want to understand.... if they get bogged down by stats & CS, it seems apt; but the sign up rates & drop rates would merely indicate a demand for the philosophy regarding ML, AI, & current state of CS...if the courses offered are limited & people want to learn, they are going to get attention...