Because BGA is adapted well to low nutrients, like a smaller critter needs less than a much much much larger one(plants).
Plants can use the nutrients much better when the concentrations are higher, BGA can when they are lower(but they max out their uptake at a lower concentration as well while the plants keep increasing the uptake rates).
It has nothing to do with air N2 sources, they simply don't need much, just like GW which can also easily survive on extremely little NO3..........it cannot use N2, but does fine.................our tanks are __far__ from NO3 limiting to BGA..........
Grow the plants, the algae will no longer grow well.
But.......something will grow, you have a choice of what.