Because they are lab tested and tried and true. They also match multiple other kits. They are considered the gold standard for reef keepers.
I didn't know that Hanna is the gold standard. Wonder if the other test kit users know this.
Meanwhile most trident tests don’t match to other test kits. My trident is ALWAYS 5-10% compared to every other test kit. Including a recent ICP I sent out. The ICP matched my Hanna tests exactly.
You shouldn't chase numbers across kits be it manual, automated, or even ICP. But if that is your jam, great. But just for fun I have numbers too. I even have Hanna numbers to compare with but didn't want to go dig those up.
Edit: ICP (ATI) date, Trident (test time)