I really feel its a lack of resources to do it mixed with song complexity. You can find Midi's and sheet music alot easier for older games than you can for new ones which means you have to remix the melody by ear and that can seem intimidating to remixers I think, especially new ones. On top of that songs nowadays already have a crap ton of layers to them and a fusion of multiple genres which makes it harder to think of new or interesting ways to remix songs.
This is just my thought on this...