Except the plot of the Mass Effect games have always revolved around humanity.
Humanity != Earth. Two of the three backgrounds for Shepard have her growing up off-world.
whoopy do its still the planet with the biggest human population, so you kind of have to save it unless you want most of humanity wiped out.
And also when ever they use the humanity is threatened angle in these games they always mention Earth as being threatened such as Anderson stating that even Earth would be a target for Saren, or speculating that the Collectors are going after Earth based on how many people can be held on a Collector ship, or how Shepard points out how by using the Alpha relay the Reapers could hit Earth once they arrive.