Moms, religious groups, game rating, lunatics
Think that covers the reasons why no evil alignments. D&D is always being carefully watched now either by one of those things or because of one of those things. Why was LOTRO advertised on tv when it went free to play but not ddo (at least where I live). Until some lunatic hacks up his family in order to get more time to play LOTRO it is still safe to advertise that game on tv without backlash. Adding evil alignments is a good way to draw out the backlash that is always lurking and waiting for a chance to attack anything with the D&D name on it.
Lithic covered the gameplay related reasons.