Earth is no longer habitable.
Humans narrowly escape Earth using tried-and-true methods of the Apollo and Shuttle programs. Although newer technologies were employed, they were untested and many people lost their lives. The resulting technology in the Venus Rises world is a mishmash of newer and older science. Generally Venus has the older tech, their ships are clunkier and assembled from scrap parts. Mars, which has the R&D budget, produces more streamlined vessels with progressive technology.
Mars is the home of the social elite and Mega-corps.
When Earth was evacuated, it was the multinational corporations which had the resources to escape environmental and geopolitical collapse. The Board of Directors of these companies became the ruling class of the newly formed Mars Defense Directorate. These companies strip-mined the moon for water and other minerals necessary to establish the first colonies on Mars.
Venus is where the mining colonies and workers reside.
There are 26 colonies on Venus established by various competing companies. These companies set-up colonies on Venus to house their workers. Miners are promised a comfortable retirement on Mars pending fulfillment of their contractual obligations.