Because, for now, those non-nuclear-capable states weren't interested in becoming nuclear-capable. On a standard PWR (the most common type of civilian industrial reactor) cooking military-grade plutonium is easy: charge it, let it start ('diverge') and then run, all as usual, then shut it down early.
How many nuclear electricity states are there? 30
How many nuclear weapons states are there? 9
What headaches are those nine nuclear capable states providing, exactly?
How has the world been made worse by having nine nuclear capable states? Practically, not just hypothetical anxieties about an unrealised future.