Root obviously skipped the tri-series because he was very confident that he would get picked for the IPL. If you look at it from his perspective (with the assumption that he was going to get an IPL contract), there was no point of playing in a week long series, when he was going to play in a two month long tournament of high quality T20 cricket with a lot of money on the table.
Playing 2-3 matches over a week is not going to improve the T20 skills of a player. However, if you want to really improve your T20 skills, the IPL is the place to be. A two month long tournament with a gruelling schedule against a variety of quality sides. It really does not get better than that. In hindsight, it was a poor decision by Root, as he overestimated his value to the IPL franchises.
Secondly, the amount of money Unadkat and Tye fetch has nothing to do with the prestige of the league. The IPL is the gold standard in franchise T20 cricket because it has lead the way for other leagues who are nothing but poor imitations, and it has been around longer than the other leagues. It has the largest fan base and is at a different level in terms of popularity.
That is what makes the IPL more prestigious than the other tournaments, not how many millions player X and player Y get.
I do not agree that if all leagues achieve parity in terms of the financial incentives that they provide, players would choose the BBL. The BBL is better than the PSL, CPL and BBL, but it is insignificant compared to the IPL, a league that has developed its own history now, and winning it actually means something. It is the only league where there is genuine pressure on the players to perform. It is not a paid holiday for pensioners and other dead cricketers.