It's not directly correlated with making the playoffs/not making the playoffs, but your record from the season before always has a pretty strong impact on next season. I.E.- making it to the playoffs doesn't get you that boost, but the fact that you win 18 (or whatever #) of games to get there does. In theory there's a loophole because you could win 16 games (or whatever) and miss the playoffs and still benefit the same as a 17 win team that did make the playoffs, but that's almost a moot point.
It's also not a huge boost, but I can't remember ever having a playoff game that wasn't sold out, so there's a minor revenue boost there as well.
To note, I'm not disagreeing with you at all, in fact I agree with your proposal, and I especially wasn't a huge fan when playoff revenue split became 50/50, this is just my thoughts for the rationale behind it all.