When the First Doctor encountered the culture, it was clearly in decline, having all but lost the evolutionary war on Aridius to the Mire Beasts. The Mire Beasts had virtually uncontrolled growth, which had pushed the Aridians to a tiny, hidden stronghold. The Aridians were led by a ruling council of elders, whose orders could not be challenged by their juniors. (DW: The Chase)
The Aridians originally lived in an underwater city, but the oceans slowly dried up and turned into deserts. The Aridians had to not only adapt to this new climate, but also contend with the Mire Beasts and a brief Dalek invasion when they landed their time machine on Aridius in search of the First Doctor and his companions. They chose to collaborate with the Daleks and surrender the Doctor and Barbara. (DW: The Chase)
Behind the scenes
According to non-narrative information in DWBIT 41, many years after their encounter with the Doctor, the Daleks attacked Aridius, as it reminded them of their failure to catch the Doctor. After the destruction of the Mire Beasts, the Aridians were left undefended and easily exterminated.