I can apply for loans, communicate with doctors, and do any number of other sensitive things online, but to vote I must go somewhere (which may be close to where I live but far from where I work) and wait in line, or send a bunch of stuff through the mail. These transactions costs are just as real as any other costs, so theoretically their elimination would be an unambiguous win for society.
The cynic might say that the nonexistence of online voting is deliberate. If online voting is allowed, the demographics of the turnout would be much different (i.e., more busy young people), which might be to the disadvantage of those in power. It's easy to see that the most passionately political people and those with the most spare time or the shortest commutes are overrepresented in the final tally. Whether this is good or bad is an open question.