This is because of a few reasons. Humanity was devolved after our defeat during the Human-Forerunner War to the point where we lost hundreds of thousands of years of technological progress, and tens of thousands of years of genetic evolution. The other Covenant species had no such event happen to them. They were allowed to keep their intelligence and technology, even after Halo was fired. Of course, the Librarian planned to rectify it. The majority of humanity was stored in Forerunner keeping for safety from the Flood and Halo. implanted with powerful geas designed to advance humanity faster than normal. Among these humans were also those who redeveloped old technologies, like basic engineering, steam engines, and gas powered airships. To put it simply, humanity was set up for rapid readvancement and greatness. That is until the Didact composed the majority of humanity in the Forerunner's possession, leaving the Librarian with a handful of specimens left on Earth. These humans were not taken because they did not fit with the Librarian's plan. With so little humans left in Forerunner possession, the Librarian quickly altered her plans, and returned to Earth, to gather the sick, weak, and unfit in order to ensure our species' survival. With her plans torn apart by the Didact, she ordered the construction of a gateway portal to the Ark in order to give humanity direct access to the Forerunners' last home.