Why is the matter-antimatter asymmetry of the Universe quantified by the baryon asymmetry, $$Y_B=\frac{n_B-n_{\bar{B}}}{s}$$ i.e., the difference in the number densities of baryons $n_B$ and antibaryons $n_{\bar{B}}$ normalized w.r.t the entropy density $s$. What about other matter-antimatter asymmetries such as difference in the number of leptons and anti-leptons?
Answer
People do consider the lepton asymmetry, it plays an important role in Leptogenesis. This is the idea that the baryon asymmetry was "born" as a lepton asymmetry, which was transferred to the baryon sector during the EW phase transition.
Having said this, the baryon asymmetry is certainly discussed more often. This is because 1) we don't know what the lepton asymmetry is today, because we cannot determine the composition of the neutrino background, 2) we are made of baryons (and some leptons, which come along for the ride), so we would not exist without a baryon asymmetry (but we could conceivably exist without a lepton asymmetry).
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