I saw a documentary on this recently and what it boiled down to is that they want to but are nowhere close to solving the many and varied problems.
Nine months to get there and nine months to get back plus time on the surface which will vary between a relativeley short time and a few months. I don't understand the science/physics of why that must be but its all to do with the moving relative positions of the planets and orbital mechanics.
Eighteen months plus of food for X number of astronauts presents huge weight problems to say nothing of the need to carry the associated rocket and fuel to get back.
Even if they carried everything needed in multiple trips to a space station and launched from there, with existing technology the cost would be astonomical (no pun intended) compared with anything that has been done up to now.
In summary the program gave the impression that such a trip is many decades away.