First of all because the problems concerned with the security of the net are increasing considerably. While at a polling station there is a visual and physical "identification", via Internet there can only be an electronic "identification" and it's therefore possible that someone else simulates to be you and votes in your stead. Moreover in areas with a large presence of a strong criminal underworld, (where there is none?) , it'possible that somebody comes to your house and makes you vote as he likes, by the use of threats. Without having to enter your house somebody could be content of a visual recorded evidence of your vote ( who hasn't a video-recorder nowadays? ). The presence of criminal underworld in the social structure is a further reason, and not the least one, why to go along with the paper system over which a larger democratic control is possible.