Aza-enolates: why no reaction at N

My understanding is that aza-enolates have an advantage over enamines in enolate-type alkylations because the reaction at N is less likely. As a result, unactivated electrophiles can be used (i.e. electrophiles that are not allylic / benzylic / $\alpha$-halo, source: *Clayden et al., p. 593*)


Question: why is the lone pair on N in aza-enolates less reactive? 


