I think there are probably many possibilities. One could be the strength of the lighting in relation to the depth of the tank. Maybe because they are at the bottom they are not getting strong enough lighting so they attract algae because they're not thriving.
Other than co2 and circulation of co2, I agree with growth rate. I I had bba on hc but had none on my glosso and they were next to each other in the same tank. Glosso grows faster. With that said, the bba only grew when I was having co2 troubles.