Mars's importance had plummeted, largely because humanity had realized through continued research that they did not need to develop ground-based engineering machinery first like ordinary civilizations. Their technological foundation was already sufficient to skip that stage and move straight into developing space machinery.
Under normal circumstances, a civilization needed to use a planet other than its homeworld to test all kinds of ground-based equipment. They had to first build an engineering base on another planet, slowly gathering everything required to manufacture starship components. Over the many years to come, that base would produce all the starships the civilization needed, as well as all kinds of machines for use on other planetary surfaces.
But humanity did not need to do that. Starship components were produced on Earth and Mercury. The nano-compression technology mastered by Stark Industries allowed them to produce the most equipment in the smallest possible space. They had directly turned Mercury, that energy planet, into a production base.
The assembly stage, which required the most space and was also the most troublesome, was instead the simplest part for humanity. They had both mages and Mutants, so they could assemble things while flying through space. There was no need to painstakingly put everything together in a gravity environment before taking off.
Then there was engineering machinery. A certain Eastern Great Nation had already maxed out that branch of the tech tree in order to produce faster-than-light engines. Once they left Earth, they could directly mine Jupiter without having to find another place to build a base for producing machinery used to extract resources from gas giants.
Thus, Mars, the ideal stepping stone, had become a tasteless chicken bone—too good to throw away, too pointless to keep. There was no great need to use it as an industrial base; if they