I associate part of the loss of interest to do with there being a big technological step needed to get to the next stage, although you could maybe think of it as smaller steps, in most peoples minds you get into space, you land on the moon, then you land on another planet (or maybe a moon of another planet) landing on the moon was relatively easy compared with getting to another planet.

I would assume that once space travel starts to take off then technology will quickly advance to make it quicker and cheaper. Although the vastness of space is a huge obstacle to overcome, outside of wormholes or maybe some kind of suspended animation it would take generations to get anywhere, unless its possible to travel faster than the speed of light...