The passengers were put into suspended animation to prevent excessive aging during the 120 year trip from Earth to Homestead 2. They are going to start a new life. Dying of old age on the ship halfway there would make that difficult. The point of homesteading, and the reason it's so profitable, is that people with the skills or money to make it there want to live it out, there.
Of course, supplies is a secondary reason. Enough food, water, and oxygen for 5000 people (plus any births) during 120 years would be an excessively unrealistic option on a completely contained ship. Even with the amount of trees (which also seem to be in suspended animation) and supplements and water recycling, it would be too much.
Although they seem to have a significantly powerful energy system (they are able to travel at 1/2 C, have a force field, etc), it just wouldn't be pleasant for everyone to live and die in a (luxury) tin can.