I ran across this at gotfrag.com. They might be onto something here. Here's a snippet:

"...on the Smash Bros Dojo site...there are songs from each Metal Gear Solid game. The twist? The primary song (first listed) is a piece from Metal Gear Solid 4. Why would that piece be included? Every single song on the list is from a game that has appeared on a Nintendo system, or will soon."

So there you go. This is just speculation but you never know...