There are a load of reasons why space development slowed down compared to during Apollo. The short version is blame Nixon. (and advancements in robotic technology making manned exploration less lucrative).

Nixon decided to try and normalize NASA, changing it from the sort of special status it had enjoyed during the space race to just one more government entity competing for funds. He saw the US presence as space as merely being part of its domestic policy rather than a scientific endeavor. As part of this he made the decision to focus on low earth orbit instead of going further out into the solar system.

The sharp reduction in funding (adjusted for inflation, NASA's budget is half what it was during Apollo) and a somewhat murky vision of what the next step was after Apollo meant that there was fierce competition for designs.

The idea that gained the most attention was to create a reusable spaceplane that could make large numbers of LEO missions for cheap. This would eventually become the space shuttle, but the initial plans were quite different, envisioning the craft as much larger and having it piggyback on a separate launch vehicle that would land separately. NASA went ahead and cancelled the last several Apollo missions for the purpose of saving money to construct these.

At some point in the design process, NASA was desperate for support and teamed up with the Air Force, who had just had some of their space projects cancelled and were interested in the shuttle's possibilities. The Air Force eventually wound up taking over a large portion of the design process.

The result was that the shuttle was given much larger wings than it needed as well as an expanded cargo bay. Further budget cuts meant that the fully reusable booster vehicle was changed to semi-reusable booster rockets and an expendable fuel tank. The booster rockets were further changed from liquid fueled to solid fueled because they had a cheaper development cost (but higher per-launch cost and worse environmental impact). The shuttle had been envisioned as a super-cheap solution for getting to space that could potentially do a lot of what the Saturn V could, but in the end they quibbled about the initial cost so much they cut out a lot of the measures that would save on per-launch cost, making it more expensive in the long run.

At the same time, robotic exploration has taken off. Manned exploration is hard, and robotic exploration is easy, and accomplishes a lot of the scientific goals we have without the added complexity of a manned mission. There are still reasons to go on manned missions, but robotic exploration took some of the wind out of its sails.

And finally, space exploration isn't a straight difficulty curve. Manned exploration gets much more difficult as duration and distance goes up. A trip to the moon is just sticking three guys in a tin can for a week. They're back before any long term problems come up.

Going to Mars would take years, meaning they have to pack enough supplies to last them years, and deal with the effects of zero g and increased radiation for years. Our long term astronauts can't even stand up for the first few days after they return from the ISS and need rehabilitative therapy. Mars' gravity is only 1/3 but they'd have to deal all on their own. And down the line, if they want to come back, they need a much bigger escape rocket than we had on the moon. And they need to prep and launch it with no help.

Plus, they'll have to do all of this with a 30 minute delay in communications with Earth. NASA babysits all space missions to an absurd extent, but they won't be able to backseat drive for a manned Mars mission.

TL;DR Nixon cut the budget and the scope of the program. Shuttle era cost more than projected and was limited to LEO. Robots are cheaper than people. Putting people on Mars is a thousand times harder than putting people on the Moon.