Nearly same here, 2 years behind you. 83+ had just come out which I think added some Flash memory for Archiving and installing ASM apps (mostly games is what we used that capability for). 85 was out there but uncommon, and the richest or smartest kids had 89s, which were and still are an absolute beast. It blew me away watching people solve equations and simplify expressions on that.
To answer OP though, I think the reason the 84+ (which is or just emulates the old Z80 goodness of the 82/83/83+) is still wildly popular* is that more advanced calculators can easily do a LOT of stuff for you -- right out of the box -- that you're ostensibly there to learn to do yourself, which brings into serious question why bother taking the class in the first place. So teachers would prefer kids to bring a less overpowered calculator to class.
An 89 is basically to say, Calculus AB as a standard 4-function calculator is to 3rd grade math.
None of that is a knock on any of those calculators, though. It's incredible what they can do!
* Let's all take a moment to appreciate the genius of TI repackaging the same 1970s technology in a shiny new case every few years and getting away with -- STILL to this day -- selling them for $150!
To answer OP though, I think the reason the 84+ (which is or just emulates the old Z80 goodness of the 82/83/83+) is still wildly popular* is that more advanced calculators can easily do a LOT of stuff for you -- right out of the box -- that you're ostensibly there to learn to do yourself, which brings into serious question why bother taking the class in the first place. So teachers would prefer kids to bring a less overpowered calculator to class.
An 89 is basically to say, Calculus AB as a standard 4-function calculator is to 3rd grade math.
None of that is a knock on any of those calculators, though. It's incredible what they can do!
* Let's all take a moment to appreciate the genius of TI repackaging the same 1970s technology in a shiny new case every few years and getting away with -- STILL to this day -- selling them for $150!