good effort, but some suggestions
Nice job pulling together the numbers.
1. Some of those winning percentages seem way too close to draw the strong conclusions you're making. I suspect some are not even statistically significant (differences could be chance alone). Is the SEC the best? Perhaps, but the difference between a .559 and .553 doesn't prove that.
2. As some have pointed out, sheer numbers of bowl games is pretty misleading. For example, until the mid 70's or so only ONE pac ten team was allowed to play in a bowl game. Didn't matter how many wins you had or what you were ranked, you didn't go anywhere if you didn't win the conference (in 1967 UCLA went into the USC game #1 in the country and unbeaten, but lost and didn't go to a bowl!). It wasn't until the 80's did more than two teams play consistently in bowls.
3. There is an obvious difference in conference size that also skews numbers. You might consider normalizing by number of teams for the current data.
Finally, I put no stock in the the BCS titles that you keep referring too. Without a playoff those games should be considered just another bowl game in my opinion. It is mostly a popularity contest