The first real group of people to explore the idea of space and the exploration of space through technology was created in 1924. Created by Konstantin Tsiolkovsky and aptly named The Society for Studies of Interplanetary Travel, it had 202 members, merging space and rocket experts into one group. Tsiolkovsky was also one of the first academics to publish papers exploring the use of rocketry for reaching space in 1903. Back then the idea of reaching space at all would have been revolutionary. However, the development of this idea happened relatively quickly after that. In 1926, Robert Goddard launched the first liquid-fueled rocket, with the Spaceflight Society being launched in Germany the following year. In 1935, two years after the first liquid-fueled rocket was launched in the Soviet Union, Emilio Linares in Spain designed and then created the first fully-pressured astronaut suit. The Russians later used a model when going into space, the Americans also then adopted Linares' model when creating their space program. His model astronaut suit has been the basis of most astronauts' suits when going into space.
You may be wondering why this started with a congested history lesson. But that barely covers the evolution of our relationship with space exploration and technology since the start of the 20th century. From 1902 to 1969, the biggest gap in years between another development in space technology or exploration is 3 years. But then there is a massive gap between 1969 and 1981 where no real development happens. And then there is an eighteen-year period between that development and the next in 1998. So why does the previous history of evolution possibly contribute to the lack of high-speed developments?
We've possibly just grown into space technology which means slower, less big breakthroughs or developments. This has been seen in other fields such as medicine, but in recent years there have been bigger breakthroughs such as phage therapies to treat antibiotic-resistant bacterias, whereas astronautics and aeronautics have plateaued. As we grow more comfortable with space travel and the technology behind it, we risk the danger of growing too comfortable and falling into a permanent plateau.
Another reason why we may have slowed innovation in the field is the fact that technology itself may not be able to keep up with astronautical innovation. You can only innovate so far as you have the means to, and if you don't have the materials, without engineering them yourself there's no way to develop new space technology.
This isn't to say that no breakthroughs have happened recently. In 2012 SpaceX Dragon capsule was the first privatised spacecraft to dock with the ISS. SpaceX's Falcon 9 Flight 9 in 2014 made the first ocean-soft touchdown. In 2017 SpaceX reused an orbital rocket, with a successful return. In 2018 New Zealand launched their first rocket, also taking up a carbon fibre sphere that now reflects the sun's light, possibly helping with the environmental impact. But if you notice the pattern there, most of the innovation has been done by a private space agency under Elon Musk; which could say something about the funding that government space agencies get.