It's a matter of point of view. DRM technology is sold by security vendors to content vendors (distributors, not necessarily producers). The interest of content vendors is to limit the distribution of the content, hence what they buy is confidentiality of the content. Availability is not important to content providers, except as a minor point (too little availability may anger customers).
Availability is in the interest of the consumer, who often has no relationship with the DRM provider. Even if the DRM provider is providing a DRM-aware reader to a consumer, availability is a central functionality of the reader, but it is not generally a security matter: unavailable content is just an application bug. For availability to matter, there would have to be a critical concern that DRM-protected content remain available in some circumstances. An airplane entertainment system is the one case I can think of where content availability is mission-critical (but to the lowest degree, only inasmuch bored passengers get cranky and are harder to handle).
DRM today is not directly concerned with integrity. If the consumer wants to view external content that has not been vetted by the content vendor, this is usually allowed (e.g. to allow people to view their home-made content). There are several ways in which integrity can get involved, however. If a device is restricted to only displaying approved content, then integrity does matter. Furthermore, under the hood, DRM content relies on the storage of access rights. If the consumer can modify those access rights, or influence them (e.g. by resetting the clock at the end of a rental period), this affects the security of the DRM implementation. Thus, even though DRM itself is about confidentiality, the implementation (usually) relies on confidentiality. (This is somewhat similar to the way authenticity often relies on a secret key, so even when the data itself is not confidential, confidentiality is involved with respect to the key.)