Older Bioware games did allow you to play as "evil" within the storyline. Of course, that was before voice acting for every interaction with NPC's. Voice acting and scripting takes up such a large portion of the budget, and other resources, that it restricts many choices, including an evil storyline,
The oldest Bioware game I've played was Kotor. That only allowed you to play "evil" at the end of the game - and I'm fairly sure that was only because a sequel was never intended. So unless things are very different in Baldur's Gate, NWN, or Jade Empire (none of which I've actually played, myself, so I could very well be wrong) my guess would be that the games which gave you the "evil" option didn't let you play "evil" the full game. Maybe you could play a total jerk for 90% of it - but not completely moustache-twirling "I'm going to rule the world/bathe the starways in the blood of the innocent and righteous" evil.
The vast majority of gamers don't actually like to play "evil" unless its in a campy humourous sense ala Dungeon Keeper. Once its presented in a serious sense few people will play it but now you're doing things and making decisions that are legitimately immoral or unethical. The fundamental problem is that in a serious game its extremely difficult to give a player a motivation to be evil. Because no matter the reason you give the player they can reason there way out of it to some degree and you can't just tell them well you're evil because you're evil.
Its just human nature to be honest.
You could assign the role of the villain to the PC and leave it from there though. Every Dragon Age game assigns you a role ( The Warden, The Champion, The Inquisitor ) and then leaves it up to you to navigate. So you could play it that way. Assign the role of the villain to the player and the world views them as a villain. But have their objective be something actually good or heroic that they're trying to complete while struggling against the stigma the world is imposing on them. That kind of role would be loaded with RP as you try to convince people of your intentions or utilize more unsavory elements of society to further your objectives. Ends justify the means, etc.
Very much this. You could argue, however, they've already done this in Origins and Inquisition. Origins; Loghain has most of Ferelden convinced you killed the King and are a menace to society, up until the Landsmeet. Inquisition; the Chantry declares you a heratic, and blames you for the death of Justinia for the beginning portion of the game. Sure, you're seen as a hero at some point in both games - but I feel like this is kind of a given. If you start off with people thinking you're a villain, and end the game with people's opinions of you not changing in the slightest, then where's the change? If the world doesn't change in some way from beginning to end - if it just stays stagnant - then what is the point of any of it?