I believe Skip Williams once stated that the reasoning for 2:1 rather than 1.5:1 was because people generally hate dealing with the decimal point. While myself and probably many people who care enough to post on a D+D rules message board are generally math people who cringe at the thought of making something more powerful because others can't multiply, I can testify to the fact that there are gamers like that.
I believe the math for 1.5:1 does make the Power Attack feat a little better balanced (I think someone did a nice spreadsheet once), it isn't a huge difference. On the other hand, 1:1 almost never works out to a benefit, and only works when you have an almost guaranteed hit.