Why can some people use certain songs/clips on YouTube and I can't?

For example, I wanted to use a Led Zeppelin song in a video, and when I posted it, it was either deleted or blocked in some countries. I saw someone else's video (not official or Vevo), and they had the same exact song. Same goes with movie clips. Why can't I use them, while others can? I have heard the idea that it's if it is more popular, they'll let you use it but just add an advertisement. If they remove it almost immediately, how do they know if it will be "popular"? Please help? Thanks!