The sobering answer is, because most people couldn't care less.
No one has walked on the moon in 35 years. We are further away from the Apollo program than Apollo was from the great depression. Mars is a distant unknown date on NASA's calendar and private space flight amounts to little more than barn storming. The fact is spending the necessary money to explore our solar system is simply a matter of priorities and space exploration is not a priority.
Some would ask, “Why explore space at all?” The only answer this question needs is another question. Why explore anywhere? The answer to that question is going to very from person to person but as soon as you start parsing the answers you will discover something. Most people are not explorers, nor do they recognize the importance of exploration. They will tell you they don't see the point in spending money “up there”. Never mind the fact that all the money will be spent 'down here' they just don't see it.
Many would let machines explore space because it is too dangerous for humans. Don't worry they will. Machines are getting smarter all the time and if they ever surpass us in intelligence then they will inherited the destiny we thought was ours. There can only be one "first step", let it be ours, let it be human. The Universe belongs to us. For now.
Maybe the government should boost spending until private industry can take over. There are no voters in space so politicians as self serving as they are, are not going to fund space exploration. And most people being as self serving as they are, are not going to demand that their politicians spend more money on space exploration. There is a unhealthy relationship between NASA and the U.S. Government. NASA pretends to explore space and congress pretends to fund them.
Space exploration is expensive, you can't do it on the cheap. NASA still hasn't learned this lesson. They intend to retire the space shuttle and even hand the space station over to the Russians to free up funds to build the kinds of rockets we launched 30 years ago. Of course they'll lose the shuttles ability to do repair and assembly missions and the space station as a staging location but they are stuck. Congress just isn't going to come up with money for an energetic manned space program. NASA and every other space agency in the world is underfunded. If it were possible to do more with less then other countries that are talking big and spending a lot less would be going further. No one is where NASA has been because no other government is spending the money the U.S. government has in the past.
Let private industry pay for it. Private space programs are in their infancy but so far no private billionaire benefactor has decided to spend his entire fortune and go to the moon.
Would it even be possible for private business to explore space? They're are no customers in space and there aren't enough people able to drop 20 million dollars on a ride into orbit to make it pay. Let's see a 10 billion dollar a year space program would require 500 people to spend 20 million every single year and risk their life to sustain it. Nope ain't gonna' happen.
Space exploration is going to have to wait until there are literally hundreds of thousands of billionaires. Who could spend 20 million like it's pocket money. That's going to be a long long time.
Of course where there is a will there might be a way. I wonder what that way might be?