Because college football is governed by a system that is largely based on perception, opinion and style points.
When it comes to bowl season, it is not as much about the individual teams anymore as it is their conferences and the perception of that conference relative to their competition (i.e. BIG vs. SEC). This influences national rankings which influences the CFB, selection committee bowl placement and overall opinion of the "strength" of the conference.
Case in point - Cincinnatti is undefeated and ranked at #4 behind a bunch of 1 loss teams. Mathematically they are in first place. But because they are a part of the AAC and not one of the P5 schools, their record isn't as signficant. Had they lost a single game, they may not have been in the top 10 let alone in consideration for the CFP. Another team from the AAC, the Houston Cougars, were 11-2 on their season and ranked at #20 behind a bunch of 3 loss teams and one 4 loss team. Based on their record they should have probably been in a higher bowl game than the Birmingham Bowl, in which they played a 6-6 Auburn team from the SEC and won by only 4 points.
Purdue beating Tennessee helps the BIG brand, which is all these bowl games are really about.