Top thirteen reasons why the blue Na'vi people of that movie didn't build planetary defenses before the second invasion

1.  Because they're goddamn hippies who didn't think they'd be attacked again
   
   
2. They were overconfident and figured they'd be able to defeat the enemy again and again, ad-infinitum
   
   
3. They were confident they'd be easily defeated in case of a more serious invasion from Earth and there was no point wasting resource on something that couldn't be prevented
   
   
4. NIMBY's who didn't want the weapons or launch centers or the associated industry for construction near their neighborhoods
   
   
5. Massive government corruption
   
   
6. All the different tribes were busy fighting each other, they didn't get the time or resource for joint defense
   
   
7. When somebody said 'they might attack again', the defense guy heart 'they might uh tack a grain' and replied 'its okay we can deal with that, no need to prepare defence'
   
   
8. They were crazily optimistic and didn't think the day would come
   
   
9. They had signed a mutual defense treaty with a nearby satellite and nobody came to their defense
   
   
10. Humans had really good military intelligence and destroyed most of Na'vi air fleet before they ever took off.
    
    
11. They decided to not invest in telescopes and therefore had no time to prepare for a proper invasion. With proper warning their performance would have been better.
    
    
12. The big guys on Na'Vi re in cahoots with the invaders
    
    
13. The blue people were far too busy to watch MCU movies to focus on their own defenses.