Bottle-necks in technology possibly? Until we can go faster than the speed of light(there's a theory that says it's impossible), there is really no point in doing any manned space travels. It took them > half a year to travel from Earth to Mars to land Curiosity the Rover. The nearest star system to ours, Alpha Centauri, is 4.37 light years away. New Horizons, currently fastest spacecraft recorded, can go 15.73km/s, about 0.005% of light speed. Even if we can travel at 1% of light speed it would still be 437 years to reach them.

So instead of that, why shouldn't we just explore our earth? Specifically under the sea. Domes under the sea with our current technology is much more doable compared to space-faring.