(also 'not to offend,' this is just my opinion)
i think the rise and fall of 'artistic' music is in direct relation to musical theory and technology. between the medieval period and the baroque in western europe, musical theory became increasingly more complex; more pitches were added to the octave (in some lesser known cases far beyond 12, read 'archicembalo'), peoples' perception of consonance expanded, and the act of tuning keyboards was an art in itself.
past that point music became standardized; the variety of different temperaments (meantone, werckmeister, etc) were phased out for 12-tone equal temperament. 'serious' music began to turn it's focus towards the soul rather than the intellect. by the early 20th century, this one standardized system of music became so exhausted harmonically that some composers chose to abandon what traditions existed.
yet rather than develop new systems of music (new tunings/new instruments), they tried to re-work what was already understood to be old. serialism, then minimalism, then all the strange kinds of 'art' music that goes on today (pretty much unnoticed by the public).
that is why today we have a religious devotion to music from the 17th-19th century. that was the peak of harmonic development in music. and that is why most popular music today seems simplistic and trivial; people unconsciously settled for what was 'pleasant to the ear,' rather than push the envelope harmonically like what was going on only 200 years ago.
and yes, there is stuff around today that i would consider a break-away from all this...harry partch most notably, but even when rock bands experiment with tones and electronic effects...there's definitely a yearning there for something new.
EDIT: someone's probably going to say 'well what about jazz!' i think jazz introduced new textures into music, especially with the advent of the drum kit. it also offered a new intellectual way to look at music. i think it's a much bigger discussion as to why jazz came and went, i'm really just talking about the technical aspects of music.