2 Reasons:

In some cases the original copyright holder will not let us. Point in case would be Tim Story's Collected. Sony Music, who owns Windham Hill, contractually will not let us.

Another reason is that the majority of Nepenthe Music releases have been sequenced by the artist. This was their vision on how a listener should experience their release. For further reading on this topic read the blog, Rubato, for a fuller discussion regarding track sequencing and artist intentions.