Obama explicitly stopped an effort to get to the Moon with a massively ignorant “been there, done that” (exact quote!) attitude. If he signed legislation authorizing it, it was due to being part of a huge authorization bill for which space policy is not important enough to quibble over.

The brief history is that NASA has a legacy of not taking cost effective routes to exploration and rather preferring the too expensive to sustain Apollo model. The first George Bush gave a directive to get to Mars. The massive program that they came up with would have bankrupted the economy. Congress instead forbade NASA from spending any money towards people to Mars.

Clinton took the focus away from destinations and supported the ISS as a post-cold war olive branch to Russia.

George W Bush lifted that ban and gave the same goal towards boots on Mars with explicit direction to pursue a commercial space settlement approach that through cost sharing and reuse would have kept total cost reasonable and bootstrapped a self sufficient extraterrestrial economy. That directive was ignored and we got what NASA literally described as “Apollo on steroids”.

Obama killed that albatross without a replacement or any intention of providing one other than giving them permission to do something like visit an asteroid, as pointless an exercise as that would be, just so long as it could be done within a frozen budget, meaning minimal if any tech development (a directive to do tech development and a budget was provided, but it was substitutive not additional and eventually gutted anyway). Congress overruled and started giving their own directions for NASA, including the pork driven SLS that serves no purpose really.

Now Trump seems to be sending the pendulum back in the humans to Moon, Mars, and beyond direction. It’ll be interesting to see how it gets unwound by a democratic successor.

If this seems rather political, it is only the mundane politics of traditional party bases and campaign contributions. There is a lot of overlap between NASA and defense contractors, typically republican supporting, and they stand to benefit directly from an Apollo like approach. Lots of waste to profit off of in the traditional NASA model. The saddest story for me is that George W Bush actually seemed to care and DIDN’T authorize the traditional republican battlestar galactica approach but rather tried to direct NASA towards the SpaceX model of lean efficient reusable commercial tech, not government programs. But he was ignored by a zealous NASA administrator with a “NASA glory days” obsession and congressional pork. Space Policy isn’t exactly what the president should be focusing on, especially with two wars going on at the time, so presidents tend to get their directives used by others with opposing agendas.

Edit: down voters explain yourselves? I mention political parties only because the answer to the question involves politics. I tried to be explicitly non partisan about it. I worked at NASA during the second Bush and Obama years and saw directly the internal policy considerations as they were made. The above constitutes my high level summary of that.