All bowls are nothing more than dedicated brand marketing campaigns. A business decision that is part of some VP's P&L for the year. Outside of the New Year's Six, which are part of the CFP, there are no "tiers". All the rest are just independent events. A festivus for the rest-of-us.
Search high and low for a "ranking" of bowl games. You won't find anything official, because it doesn't exist.
I'm not sure where this originated, but my guess is people's natural tendencies to "rank" things based on some arbitrary criteria. Location, date, longevity and sponsor are the ones most frequently used. For example, if longevity were the primary criteria, the Sun Bowl should be tops on everybody's list. If weather is the primary driver, anything in Florida, Arizona or Southern California are by far the best bowls. Using date, the Texas Bowl on January 4 must be the consolation game of the national championship. None of these are true.
I suppose you could go by payout, but that has more to do with the sponsor than anything else. Plus, the money doesn't go directly to the school. It goes to the conference, and they decide how to divide it up amongst the member institutions. The Outback Bowl had a most recent payout of $6.4M. The Citrus and Alamo Bowls payout $8.5M and $8M respectively. The Rose Bowl and Fiesta Bowl - $4M each. So the Rose and Fiesta must be lesser games. Not exactly.
My suggestion, just accept it for what it is worth. These are big money events and the BIG stands to net nearly $80M or more this year. Their motivations are simple - place as many teams as possible, spread out brand exposure and maximize revenue. That has more to do with placement than anything. Fans can squabble about which bowl is better, but the truth is, it doesn't much matter outside of playing for the national championship. Personally, Phoenix is a substantial upgrade from Nashville, on par with Florida and better than Vegas IMHO. I was dreading the Music City Bowl - multiple days of country music which I cannot stand and a home game for Tennessee. No thank you.
Go Gophers!