It happens in every form of art ever invented. First the tinkerers discover the art form, aided by a new techology. Then come the dilettantes, people who figure out the rules and structures of the medium. Next the amateurs refine the techniques, pushing a fad into a form that has commercial possibilities. Finally, the professionals—people who make the art for a living—push everyone else out. Eventually you wind up with nothing but the last two groups—and the amateurs that remain do it because they love it, but have chosen some other avocation.
Via Sullivan, the Atlantic's Alexis Madrigal has woken up to this pattern as it applies to YouTube:
When it comes to putting together a really great video and ginning up a global viral push, people with resources are winning.
And it's actually been this way for a while. While, for example, 2007's "most memorable videos" featured several homemade videos of people and cats, as Marshall Kirkpatrick pointed out years ago, the most played items have long been music videos from major label artists.
So, after a brief flowering of user-generated online media rivaling the scale and reach of professional online media, we've seen a retrenchment of traditional media structures. Sure, millions of people still have blogs, but the bulk of content that's read is produced by a small number of people who do this for a living (inside completely retooled media companies).
Anyone paying attention knew this would happen, and quickly.
Think about it: if an art form has commercial possibilities, then basic natural selection will favor people who can devote more time and resources to it. And if you spend most of your waking hours on art, unless you're seriously untalented you'll eventually become much better at it than people who only spend an hour or two a day.
As Madrigal says: I think that's a good thing.