Because Lucas wanted it that way, and others have continued the trend - plus they might lose sales if the "good" side did not win.
Have you not heard of Darth Reven?!
Bull****. Your view of "good" might be different from my own - and thus your "good" prevailing might be my "bad" taking control.
1. :D, you ignored the whole idea of the post. There are people out there that will always argue the Dark Side is stronger. You could answer any argument to any story by simply saying (The Author Wanted it that way!) Doh
2. Darth Revan is an iffy subject. It can be argued that he was both Dark Sided and Light Sided, or if you play the non-canonical KOTOR just Dark Sided the whole way through. Even with his power as the Dark Lord he still had a weakness. Bastila and and some Jedi borded his flagship and fought him, and due to the (As I mentioned Earlier) Sith lifestyle, Revan's not so loyal apprentice betrayed him.
3. My view of good is perfectly obvious. A republic/democracy instead of some Sith Emperor who believes if you can't fend for yourself you should just take a blaster shot the head and save someone else some trouble.
It doesnt matter he still was the dark lord at one point in time.