Because in basketball that Cinderella team doesn't take a chance from a legit title contender to compete in the tournament. And that Cinderella team has to keep on striking gold to keep advancing. Purdue just had to find lightning once to make it and most likely will get blown out the next game.
And in college football, because the playoff has so few teams and games, it'll water down the tournament.
You really think an 8-4 Purdue team would deserve to get in over an 11-1 OSU team who's only loss was to Michigan? Purdue lost 24-3 to 7-5 Iowa. Nobody believes they're a top 4 (or top 12) team in the country.
Only team that would love that is the team that gets to play Purdue instead of Michigan in the first round.. nobody else would watch because they'd be 3TD underdogs.
Like I said, only way it works is if conferences scrap divisions and guarantee they're getting the 2 best teams in the conf championship game, even if it means a repeat game.
Even then we still need some conference parity. All 10 conferences don't deserve equal representation..the #1 team in the Sun Belt is most likely much worse than the #2 team in the SEC.