Are you asking why did tests stop happening in 1998? Or are you asking why the video stopped at 1998? The answer is the same to both. One slight correction to the vid...Naughty Korea has set off two nuclear bombs since 1998.
A cool thing about that vid is 1963 when the first test ban treaty was signed, eliminating all tests except underground. Notice how it completely stops everyone for a month or two and Nevada picks up real quick, but USSR doesn't for almost a whole year. The reason for this treaty was worries of Fallout. This 1963 treaty drastically reduced fallout as seen in this graph:
