The explanation I remember for why all liches are evil is they become frustrated at their mental stagnation, being unable to learn new things. If all liches become evil for this reason, should all adventurers who reach max level face the same problem and limitation? Perhaps, though I would not take that as canon. The process of becoming a lich involves a number of deeply evil acts. Thus, few (if any) good mages follow it and retain their scruples should they complete it.

— Mike Mearls (@mikemearls) June 6, 2018