Early in this century, many cities and universities could support telescopes large enough to do serious research. There were significant observatories even in the less accesable parts of the world. Astronomy was very much an international science, and the IAU was founded to aid this international outlook.
In the middle of this century, astronomy changed in two ways. First, the frontier research turned to new topics. It needed telescopes too expensive for most small and many large countries. Second, physics became a more prominent part of astronomy. That left many of the existing small observatories scientifically isolated, especially in developing countries. The scientifically lonely astronomers there needed new alliances to survive.
Simultaneously, the new prominence of physics led to astronomers appearing in physics departments of developing countries. These new astronomers were also isolated and they also needed to build alliances to survive.