Space travel is not easy. Ships capable of FTL travel exist, but require huge resources to construct, not an easy feat given the War on Earth. The physics are understood by few, but everyone knows the trip is one way. FTL drives are consumed beyond repair by the voyage, and must be jettisoned to make landfall. Fortunately, though, ship reactors are not, and make excellent power sources once landside- if the ship can successfully land.
The voyage from Earth to Pandora takes roughly seven years, and each ship carries 3000 people. Additionally, banks of genetic information are sent with each voyage, vegetation, livestock and more, as cloning basic life has become commonplace.
Of the four ships to reach Pandora, only one, the first, has managed to make the landing (mostly) intact. The second sustained significant damage in FLT flight, but most made landfall on lifeboats- the ship is still in orbit, unmanned. The third was destroyed on entry, only a small fraction of its crew and passengers escaped to lifeboats- this is the tragedy of the Third Pilgrimage. The fourth and final ship to arrive crash landed on Pandora, several days travel from Carpathia- its wreckage is a toxic zone, due to a reactor breach.