Possibly because there is no definitive answer. The blues gave birth to rock and roll which spawned a lot of different styles.Throw some socio-economic upheavals in the mix add a pinch of emerging technology and you go from the Kinks to Cream and Hendrix. The innocence of the 60’s crashes out with Zeppelin a war then a big recession. The 70’s are here and things are looking grim, Sabbath then Punk and enter Motörhead and on it goes to the happy hairy 80’s days, and it’s all glam rock and on and on…. It will all evolve and new classifications added, new sub-genres invented and old ones debunked. There will most likely be a bunch of blokes debating whether Metallica are really metal in a decade.