Other mistake: Just before Patricia Whitmore's plane (no. 107) is shot down, it is briefly shown with its number mirrored.
Continuity mistake: At the end of the movie when the mothership takes off the central saucer section is in place, yet we saw the queen alien fly off in it earlier on.
Continuity mistake: Early in the film, as Jake and Charlie are towing a new weapon system to its destination on the Moon, we see Earth in the background, more than half illuminated by the Sun. Jake says they are 7 miles away and closing on their lunar destination (which should only take a matter of moments). A few moments later, as Jake and Charlie are still in approach, we see Earth in the background again, but it is now in a waning crescent, only 1/3 illuminated by the Sun. This would indicate over a week's passage of time.
Continuity mistake: When we first see Dr. Brakish Okun lying in bed the arm of the bed is up. Then just before he wakes up the arm is down. (00:19:45)
Chosen answer: It's technology that works, so why not use it? People still drive 20 year olds cars now, some because they like them, some because they can't afford a replacement. It wouldn't be surprising for the new technology to be very expensive or patented by the military and therefore not available for everyone.