Other than a variety of technical reasons already mentioned, I think VEVO is an amazing moat that would be very difficult for even the most funded of competitors to cross.
And maybe it's just me but music videos don't have the same traction that they used to have. There are so many chart toppers that I've listened to often for which I hadn't watched the video for a very long time.
Compare this to 2011 where we discovered songs, including Gangnam style, on YouTube charts.