It took 65 years to get from the Wright Brothers' first motorized flight at Kitty Hawk to the Apollo Moon landing at Tranquility Base. But instead of exponential growth, human space travel has fallen back into orbit. Rockets continue to explore space, but with sensors, cameras and robots doing most of the dangerous work.
The astronaut void does makes financial sense. NASA was consuming almost 5% of the Federal budget in 1969, but that was during the Cold War and we had to get to the moon to beat the Russians, to ensure that the space around our planet would not be owned by our enemy.
It turns out it is a lot cheaper to explore space without having to keep people alive in it. However, NASA is again pursuing human space flight and has plans for a moon base and eventually, one on Mars.
Elon Musk and his Space X company have similar goals. It turns out that human space flight is a lot more alluring than robot science.