What's the reason behind this? Is it the lack of a second superpower to compete against? Is it the much higher cost of landing a man on Mars? Is it that the timeframe of such a mission goes beyond the 8 years of a single presidency?
Yes, all of the above. And more.
Another issue is that we didn't gain much from going to the Moon. It took a ten year program to develop the Moon rocket. Know how long they estimate it would take to develop one now? Ten years. Because we didn't make regular trips to the Moon, our capability to do so has deteriorated so badly that we're at roughly the same point we were in 1959. We went to the moon six times in four years and zero times since.
Another issue is that a Mars rocket would take resources away from other projects. So some people that favor space exploration would rather direct our resources to something else, e.g. more unmanned missions or a larger space station.
The truth is that there isn't much reason to go to Mars. Yes, we could learn some things. But it was much cheaper to travel to the Moon in 1973 (back when we still remembered how to get there), and we decided it wasn't worth it. Why would we learn more from Mars? We would likely make a few trips over the course of a decade or two and stop. Meanwhile, we could have been exploring automated mining in the asteroid belt and using that to build a larger space station.
We could build a Moon base with a catapult to launch things. It could be powered by solar panels built with rare minerals mined from the moon or the asteroid belt. We could stop launching things out of Earth's gravity well and only launch people.
Right now, we can't put people in orbit and let them live out their lives. People go in orbit for months at a time and then return to Earth. Before we start talking about Mars, why not explore what we can do in orbit? Put up a real space station with rotational pseudogravity. Put enough people on it to actually accomplish something. Allow the ultrarich to visit as tourists.
Mars is a huge step with a small return. At this point, we don't get enough to justify it. There are much smaller steps with better long term returns that we could take.