Just licensing issues. Some songs are album only and not al a carte as well. Too bad, so sad. Xbox Music isn't there yet, though no service is perfect when it comes to music.
Like already mentioned above it all comes down to the different contracts Microsoft has with all the labels and artists. For the majority of music I listen to I don't run into any problems, but every now and then you'll come across something that the only may to be buy the entire album or it's not available to stream. The only thing to do is write the labels and tell them you want their catalog available for Xbox Music Pass. Won't do much, but it's the most proactive thing to do. Or you could, you know, buy the album
I have a lot of the music that I was looking for on my netbook, so I just need to sync it to my Lumia. I was just being lazy because I hate my netbook, lol.