I don't see how multiple streams (1), adaptive streams (2), or DRM (3) has anything to do with video decompression.
(1) Because your graphics card only has one (sometimes two) video decompression cores on it.
(2) Adaptive streams do edge blending so you can't tell when a high quality stream suddenly becomes a lower quality one and vice-versa. This edge blending is done as a shader (to keep frames high). You can't run shaders on the output of a video decompression core without dropping frames, so what you do instead is decompress in software and then get the shader to run in hardware. This means adaptive streaming doesn't like hardware video decompression.
(3) Because DRM requires you to protect the uncompressed video content, so you can't push raw compressed un-DRMed content over a bus to the graphics card.
BluRay disks are a special case for DRM, since the keys are built into BluRay compatible graphics cards, so in your case, you're streaming 1080p compressed DRMed content to the graphics card and it doesn't need to adaptively stream from the disk, doesn't need to send unprotected content over the bus and doesn't need to show more than one video at once.