Physics, we need some giant breakthroughs before space travel is worth spending money in. While Nasa has certainly come up with innovations, they don't have the budget or capability to realistic develop the next propulsion method, I mean, rockets, this is ridiculously old technology, unreliable, not very safe, incredibly expensive and requires a disposable method of launch.
The real reason to go off planet is to be on another planet, till we can get the taking off, landing and taking off again in the same vehicle, then there really isn't any point being out there, at all. We send probes that cost silly amounts of money to other planets, and simply things like proper communication can't be established.
We could spend 100billion a year , or a trillion, making loads of wasteful space ships, sending off probes and finding out not much at all, or wait 10-100 years, have a realistic propulsion technology, other technologys, and spend the same amount but have it actually be worthwhile.
It's epic we got a couple guys onto the moon in what is not much more than a tin can, but other than a few rocks, theres nothing much more repeated trips can do. The ability to take off, go over there, take equipment/building materials/oxygen supply, come back, and do it again, over and over, easily and relatively cheaply... theres just nothing significant that can be done.