My first experience with this was Robert Anton Wilson's "Illuminatus Trilogy." I found the complexity so fascinating. I have since gotten hooked on Robert Jordan's Wheel of Time series, Raymond Feist's Riftwar Saga, George R.R. Martin's Song of Ice and Fire, and the more contemporary additions to the Dune universe.
So now what I'm wondering is this: are ensemble books a different style of writing that has always been around? Or are they a style of writing that has evolved over time? I think back to the classics I have read, and they seem to be mostly revolving around a main character. I do remember Robert Anton Wilson standing out to me in the way it was written--- the way he jumped between storylines. Having to keep several threads in mind was a new experience for me worth noting at the time.
If it is a style that has evolved, is it mirroring our more complex and fast way of living? I hesitate to say short attention span, since these series span several books that can take months to read, but the frequent jumping around has me wondering. Or could it just be that I grew up on single main character books that my parents stocked in the house?