Star trek was based on “wave of the hand” technology. In others words, it would just be there when we needed it. Reality is not that simple and manned space travel, exploration, and especially colonization are far more difficult than the hand wavers in the 1950s thought it woul be.
The problem is twofold:
1. FUEL. Always has been and there it no magic bullet. There is no "dilithium crystal" outside the scripts of Star Trek.
2. DISTANCE. Everything is just so darn far away. And if I remember some of my astronomy...things are getting FARTHER apart.
The nearest place to get to would take a ship some 300 years to arrive to...or something equally outlandish.