1.) They know about commie/social/facists trying to run this planet and since the key word is "intelligent", they're staying away.
2.) They have commie/social/facists running their planet, and can't be considered "intelligent".
3.) Played God with the wrong genome.
4.) Got hit by a really big object.
5.) Their sun became really big and red.
6.) Too close to a supernova.
7.) Too close to a black hole.
8.) They were in the way of a Vogon Constructer Fleet.
9.) Deported all of their telephone sanitizers.