Or maybe not. China has been credibly accused of starting low-yield nuclear testing in 2020, specifically configured to evade detection, albeit unsuccessfully.
https://www.armscontrol.org/act/2026-03/news/us-accuses-chin...
We stopped because the major nuclear powers had accumulated sufficient data to enable high-fidelity computer modelling of future warheads.
It wasn't just, or perhaps even wholly, driven by environmental empathy, it was an attempt at was pulling the ladder up behind the the major powers. There are eight non-signatories and seventeen non-ratifiers of the treaty, the majority of whom already being proliferation risks.
Your stats are also arbitrary, there have been sixteen nuclear tests since the Treaty opened for signature in _1996_. Setting the cutoff to 1998 deliberately excludes Indian and Pakistani tests.