Why? Plebe, I think you are absolutely one of the best posters on here. That's why I'll pick a fight with you.
Actually, it's a discussion that reoccurs every year, the answer to your question "Why?" (select teams with little chance of winning a championship over a power conference team).
There is not one, but no less than three goals for the tournament, listed here in order of importance.
1. Crown a champion.
2. Reward success and have representation from every single conference.
3. Provide entertainment that will keep the public and corporate sponsors happy.
I'm in favor of the "tie or winning record" within conference for any team, otherwise goal number two should read: "Reward success in every single conference, unless from a power conference, in which case you can be rewarded even if you are not successful." Since no team with a losing record in conference has won a championship, you are not compromising the first goal by adhering to the second even for power conferences. The example you gave, Oregon, did not make it to the Final Four and were not remotely close to winning a national championship.
Now if changing goal #2 to "Reward success in every conference, unless from a power conference" at least enhanced the third goal, providing entertainment value, then perhaps that would justify rewarding teams unsuccessful in their conferences even though they have very little chance of winning a championship. However, the opposite is likely true. An upset by a mid-major generally provides more buzz than an upset by a team that was unsuccessful in their conference.
Thus, you neither compromise Goal #1, and enhance Goal #3, by insisting that all teams selected to the tournament have had success within their conferences, as determined by a distinctively nonarbitrary criteria of not having a losing record in your own conference. You are, after all, insisting that a team can succeed at its own playing field (with the set of opportunities its own particular conference provides), before given the chance to succeed on the ultimate tournament stage.