Cool that this thread is still going and I've managed to stay above it all and not get into a fight with anyone
BTW the answer to the thread title is:
Because music has lost it's sense of humour
People forget that Queen were funny, the Beatles hilarious, Pink Floyd weren't afraid to sneak a few gags into their early albums, Elvis Presley was always having a laugh, so was Louis Armstrong, early rock n'roll was outrageous and even serious rockers like Led Zep weren't afraid to raid the funny fridge (the Lemon Song!

) in an effort to lift our spirits.
Prince - a full-time gag-writer
Bowie - Labyrinth
The Cure - need I say more?
Modern bands and artists, by and large, though they may have a great sense of humour in real life, on the musical side are very earnest and more then a little too cool for school. This despite the fact the they are aping the work of these comedy legends!
This is why shows like the Mighty Boosh and Flight of the Conchords continue to thrive, capturing the imagination of kids and adults alike, while the music industry flushes itself down the bog with dreary guitar bands, po-faced electro-bollocks and manufactured pop-shite.