Why dysbiosis was not modeled as an main effect in HMP2 paper?

Hi! I am new for microbiome analysis and try to learn from the HMP2 paper (Lloyd-Price et al., 2019). In “Differential microbiome feature abundance” modeling, what are the considerations to exclude dysbiosis as an individual fixed effect, instead of a model like following:

feature ~ 1 + diagnosis + dysbiosis + diagnosis/dysbiosis + ..._

Thank you for your help!

Model from the paper: