For one thing, though it may not apply, is that AI has low ceiling on what they'll pay for rights. That's usually why songs aren't cleared. Those negotiations are often last minute-- like most negotiations end up.
Couple of questions.
1). When they get the rights are they getting the right to reproduce the song etc, and NOT ownership?
2). Are they getting the rights to ownership?
If 2 is yes, then for how long? In perpetuity?
It would seem that each year, their list of cleared songs would get longer. They should keep the already cleared songs (for each theme) on one list, so the contestants will know which songs they can choose from instead of being told they have to change and settle for a different song with only 24 hours to practice.