No where else needs the level of anonymity that voting does, in fact in nearly all other cases we want strong identity authentication, authorization, and accountability throughout when conducting official business.

If you can tie people to their vote you can intimidate them or purchase votes, both very bad outcomes for a democratic process. A secret ballot is a core element of a strong, stable democracy. That's a rare attribute to value to the same extent, so no technology platform has really been built with that in mind. There is plenty of active research underway to make it possible, but as it stands old fashioned paper ballots, whether done in person or by mail, have some unbeatable properties.