In the elastic scattering of neutrinos on atomic electrons, why can tau and muon neutrinos only interact via weak neutral currents but electron neutrinos can interact with W or Z bosons?
In a charged current interaction the neutrino transforms into its partner lepton and a W particle. By partner lepton I mean the lepton of the same flavour. So for $\nu_e$ $e$ scattering we get:
Muon and tau neutrinos cannot scatter in this way because it would require them to transform into an electron instead of a muon/tau.
Charged current interactions are possible between muon neutrinos and muons, and between tau neutrinos and taus, but in most situations there are lots of electrons but negligibly few muons and taus. Hence the only charged current interactions seen involve electron neutrinos.