This is an intriguing question and one that is very hard to generate an answer for. However I'm going to have a go. There are several reason why people watch music videos, some videos are powered by the song itself, however focusing on the sheer content of the video, it may be the use of SFX or it could be the use of performance from the artist/s.
MTV the giant network which screens music videos which was launched August 1st 1981 screened it's first music video The Buggles, "Video Killed The Radio Star". Is this classic the most watched music video ever!? NO. For a video that has relatively long takes and doesn't confront the male gaze theory it is astonishing how this video is still regarded as one of the most viewed videos ever. After 30 years this historic video is still being watched.
Now to what I believe to be the most viewed music video EVER. The industry now has a key primary target audience of 15-24 years of age (youthful). Laura Mulvey's male gaze theory effectively states that women are used as objects on screen to attract a male audience. Britney Spears, "Womanizer" is the perfect example of this and according to MTV is there most viewed music video. This video starts off with Britney naked in a sauna which would attract a broad male audience. The use of short takes as Britney lipsincs the song whilst dancing in revealing clothing typifies Mulvey's theory. Whilst the artist and song is targeted primarily at a female audience the video was strongly created to attract the male audience which explains the reason for the popularity of this music video.