From the official blog:

PRS is now asking us to pay many, many times more for our licence than before. The costs are simply prohibitive for us – under PRS’s proposed terms we would lose significant amounts of money with every playback. In addition, PRS is unwilling to tell us what songs are included in the licence they can provide so that we can identify those works on YouTube – that’s like asking a consumer to buy a blank CD without knowing what musicians are on it.

We’re still working with PRS for Music in an effort to reach mutually acceptable terms for a new licence, but until we do so we will be blocking premium music videos in the UK that have been supplied or claimed by record labels.