Video costs many times more to produce, and is generally consumed only once.
Music is cheap to make and people tend to listen many times over, making the try-it-and-purchase-it model a genuine possibility that video doesn't really have.
Good films have a solid replay value, i.e. you keep them in your collection to return to them later, the same way as you reread good books. And even if you view something only once, it doesn't really make DRM any more sensible than it is for something that you reuse multiple times.
For example games are hard to produce (and it usually takes a long time). Still you can get very good games DRM free, because in gaming the faults of DRM are especially apparent.
The downside of DRM - it insults paying customers, it lowers the usability of the content (restricting it to selected devices / players etc.), it also doesn't affect pirates, since the moment that DRM is broken by some skillful crackers - DRM free copies are widely distributed.
So let's see - no one benefits from DRM. Who is in downside? Paying users. So what is the point in using it anywhere?
Music is cheap to make and people tend to listen many times over, making the try-it-and-purchase-it model a genuine possibility that video doesn't really have.