Robert Cookson's @robert_cookson article in the FT is fascinating - it seems Chris Anderson's Long Tail is not happening for music online.
One reason suggested is that digital music services have to sell in a limited amount of real estate so the big revenue drivers get featured to the detriment of smaller players.
Another possible reason is Globalisation - it is now easier to sell a British act into, say, New Zealand through online outlets such as iTunes than previously. Thus international Superstars are muscling out acts that would previously have been in the local 1%.
The rising concentration of artist recorded income is noteworthy because it contradicts one of the most popular economic theories of recent years, which was famously described by Chris Anderson in his 2006 bestseller The Long Tail. Music was one of the media sectors at the heart of the book.