TV and album covers and magazines ushered it in. Elvis for instance. Not that Elvis was bad musically, but what they were selling with Elvis was almost as much about his good looks and image as about his music. I don't think it was just the female audience that was dictating that approach. You may feel they were more responsible for it than men, but ultimately we all like looking at people that are attractive (even if not necessarily sexually attractive to us in particular) more so than people that are ugly; we all would rather feel like what we are listening to makes us cool than listen to the stodgy, boring, limp music of our parents (obviously I'm talking more about teenagers than adults, but teen listening habits tend to set tastes for a lifetime).

It may have increased over the years because of MTV, YouTube, the general increase in media exposure, etc. The culmination of trends in technology and pop culture mean that the image and looks side of an artist is much more present than ever before, and music acts almost like a soundtrack to that image rather than the way you want it to be, with the image just supporting the music or not mattering at all. But the overall thrust of my argument is this: it's not new, and its increasing importance was probably inevitable with the direction media and the surrounding tech went. I'd also argue that people are more honest about it now; before we were pretending it was all about the music when for 99% of people, it clearly wasn't. The honesty doesn't really bother me. It's just another way that we've become more transparent about our role as consumers (see also: people in The Lesson trumpeting sales figures rather than artistic merit).

On a side note, the extremely anti-female nature of this whole thread bugs me. It's really not the fucking apocalypse if men have feelings and shit.