Why is mass fraction always abbreviated wt% and not ma%?
This is because of historical inertia.

Yes, you are correct that "mass" is technically more correct than "weight". However, because these are percentages, it does not matter. There are no units. A rock that has 50 weight% SiO2 also has 50 mass% SiO2, regardless of the planet (i.e. g) you are in.

Regarding using "ma%" - I have never seen it and it would be confusing, especially since "ma" will probably be interpreted as the unit "milliannum" which is weird. Using "mass%" would be better, but "wt%" is only 3 characters compared to 5 so there's the issue of laziness. I have seen papers using "mass%" and that's fine, and I wouldn't object to such usage if I were to review a paper.

That said, geologist often weigh rocks. Yes, the numbers are mass. But the action itself is "weigh". Go figure.