They believed they had no choice in the matter. Their current understanding of the technology they had created let them appear 85,000,000 years in a past timeline with what they were told was a one way trip. There were hints of other wormholes in time which led to other places but these stories were not substantiated by the premature end of the series.
In their future of 2149, the world is in terrible shape. Earth's water supplies are nearly gone, the oceans are no longer visible from space, population controls are rigidly enforced, filter masks are worn by everyone and they are huddled together using broken technology, basically waiting to die in their overcrowded hovels. Until a group of scientists discover a portal through time, into a cross-dimensional rift to another Earth, 85 million years ago.
Terra Nova is another attempt at a time travel story without full-scale time travel. The FOX promotional material describes its basic story-line: “The show follows an ordinary family on an incredible journey back in time to prehistoric Earth as a small part of a daring experiment to save the human race.”