Well, two things kinda prevented that.
1) The proliferation of bandwidth allowed folks to download/listen to a broad selection of music. This prevented a single genre from taking over.
2) There has been substantially less money being pumped into the music industry because of downloading and because folks now simply believe that they do not have to pay for music.
As a result the top selling album of 1999 sold ~10 million copies.
The top selling album of 2005 sold ~5 million, 2006 3.8, 2007 3.6, 2008 2.9 and 2009 3.2. Adele's album, sold 4.5 million in 2011, and 5.5 in 2012 making 10 million total, but that is an incredibly popular album by today's standards, and again only sold half as many copies per year as the top album of 1999. Additionally the fact that a single album was the top seller two years running speaks volumes about the lack of momentum in the industry.
This drastic drop in cash influx to the industry prevented it from putting money behind, and thus creating, a truly major (e.g. Zeppelin, Metallica, GnR, Nirvana) act during the 2000-2009 time frame. This lack of a truly major act prevented any single genre from taking over in any significant manner.
As-is the industry is in this weird holding pattern, even now. Money is really only put behind established acts or acts that are "safe" enough to guarantee a return, or that have minimal upfront costs. This is why we see tons of Taylor Swifts/Lady Gagas/Katie Perrys, etc. Costs next to nothing to take a singer, throw some electronic music behind them and then put them on the road solo with a few backup dancers. Bands are substantially more expensive to develop, produce and maintain on the road. Plus if they're cute it's a "safe" investment, due to the eye candy factor.
Kind of a shame, people that don't love music enough to pay for it, have almost killed the industry.
All that being said, it does remind me of the atmosphere in the industry in the 50s (or at least how it's been communicated to me.) Lots of overproduced, squeaky clean stars with manufactured images, created by a frightened industry. So if we're lucky this decade might be another 1960s in terms of industry changes/changes to music.