A pure democracy is indistinguishable from mob rule.
Congressmen swear to uphold the Constitution. Not that they take the oath all that seriously, but at least they generally have a better understanding of constitutional limits on government power than the average citizen. If anything, our current state of education in this country actually contributes to ignorance of the Constitution.
And if the online voting also incorporated the one-man-one vote rule so popular nowadays, the people living in the so-called 'flyover' states would, in effect, be disenfranchised. We could see demands for secession from the union from all but the northeast and the west coasts. This is also an argument against eliminating the Electoral College, but I'll not get into that here.