This is often due to the highest quality version of the feature content only being available in standard definition.
This can include content that was shot on standard definition video, animation produced digitally in standard definition, or a television program that was shot on film but edited onto SD video with the original film subsequently lost or impractical to re-transfer.
[4][5] Lossless audio is more practical and flexible on Blu-ray than on DVD, which only supports lossless PCM in 48 kHz stereo and requires a relatively large amount of data, leaving less storage space available for the video.
In contrast, Blu-ray supports lossless multi-channel audio in multiple formats and the larger storage makes its inclusion much more practical without impacting video quality.
Several standard definition concert recordings have been released on Blu-ray to take advantage of the superior sound quality available, despite there being little benefit to the video.