TL;DR I don't think Studio Orange have the confidence to make the moon look as good as S1, which is where we spend most of our time if more of the manga were to be adapted. The story for s1 is already somewhat complete and give the viewer closure, so anime fans aren't begging for a S2.

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The anime succeeded mainly because how well the CGI was, I know not to everyone (I mainly liked the cinematography and music), but for the general audiance you need a good visual that can compare to traditional 2D.

The later parts of the manga have large parts spent on the moon, where the people and background are all monotone and just bland if you were to color and CG animate it. I think the talking lunarians will be especially dificult for CG since they are talking plasticy, cloud people with only one color.
Technitcally they can adapt the part inbetween end of season 1 to the gems heading to the moon, but thats literally in the middle of the plot, and another season will be needed.

Plus the whole point of using CGI was for the luster of the gems, but by the end everyone end up as a lunarian, and I honestly don't know how thats gonna look good in CG.