Because of supply chain and logistics. You can't just go back. There are 30 NBA teams and they are constantly pumping out new jerseys now. They probably make these things way ahead of time and they probably like to have it so they arent all the same color (so teams have to call "dibs" on certain color combos). They probably have the next line of jerseys for the next 3-4 years lined up. Maybe there are mechanisms to get out, but I'm sure they come with fines/fees on top of you just having to sit on a ton of product that you can't sell. Ryan Smith has probably incurred a lot of up front losses with the Jazz and I doubt he wants to add more
Why did they get passed in the first place? Because a new owner took over and he probably liked them. He probably then took them to a bunch of yes men and they agreed with him. Then he signed off on them. He's a billionaire. If Ryan Smith asked me if I thought the things he liked where good things, I'd be like yes sir now please write me a nice consulting check. That's how billionaires have bad ideas, because at a certain point you have so much power/money/influence that everyone around you is going to try to placate you, even if you (say) you dont want that.
Also people forget the storyline about the original gradient jerseys. Remember when the Jazz couldnt even get that jersey produced because the mechanism that are used to create that jersey were more expensive/time costly to produce? If I had to guess, that was probably a big reason the Jazz changed to. We went from one of the more complex jerseys in terms of color to the most basic. There's probably a significant production cost change from the gradients to what was produced for the "rebrand".