Why we need pixel data encryption (perceptual encryption), not the complete file encryption (conventional encryption)?
The one "obvious limitation" of conventional encryption applied to multimedia data is that once encrypted, multimedia data no longer is compressible, or recognized as multimedia data (with metadata) by non-decrypting devices. This is solved by compressing before encryption, and accordingly decrypting before decompression and rendering. Confidentiality of multimedia data is a problem fully solved by conventional encryption.

There however might be practical reasons for perceptual encryption, including


We want the encrypted version to pass thru devices designed exclusively for multimedia data, including compressing perhaps with perceptually acceptable loss, or/and lacking error-correction.
We want a subset of the true data (e.g. metadata, sound but not image, a low-definition version..) to remain accessible by non-decrypting devices (which goes straight against the normal objective of encryption: that an adversary without key can not learn anything).
As an interesting specialization of the previous point, we might want that the non-encrypted low-definition version and the encrypted high-definition version share as much bandwidth as possible, which implies some perceptual model.