and why would they stop

In 1988 the US and Pakistan reached an informal understanding, which according to US officials went into effect in 1993, under which Pakistan agreed to freeze production of bomb-grade HEU indefinitely, and to refrain from enriching uranium to a level above 20% U-235. Prior to the 1998 nuclear tests, the US had reportedly obtained intelligence indicating that Pakistan had stopped production of bomb-grade uranium. However, following the tests A.Q. Khan claimed that Pakistan had never stopped making bomb-grade HEU during the 1980s and 1990s, and reportedly US officials said "we don't have enough information" to conclude that Pakistan was not making weapons-grade HEU.