abstinence from a vote is a form of social protest.

Voter turnout is fairly decent sign of voter confidence in the power of their vote to make a lasting change.

If you establish a form of mandatory voting, the state loses the feedback loop of 'non-voters' to establish voter confidence and also exerts another pressure on the citizenry; further hurting the citizens' feeling of self-control.

Basically, 'None' option or not, you're just taking another choice away from the voter.

Many tend not to remember this, but there are plenty of registered voters who don't show up or ever vote. This isn't necessarily a failure of voting advocates to get them in the booths, it may very well be an intentional protest on the part of the voter.