Bush tried to have the so called “nuclear earth penetrator” weapon developed and Congress shot him down on a bipartisan basis. He tried again a year later to have mixed yield warheads on tridents and was shot down again.
Because it’s a really ■■■■■■■ stupid idea. Nukes are to ensure MAD, to prevent warfare.
Low-yield nukes are a temptation to be used in battle, since some idiots are of the mindset that “They won’t mind if we use them”. They risk a nuclear war far more than a high yield warhead does.
You have 4,000 nukes. There is no deterrence gap, you can already ensure MAD several times over.
The idea that the US needs these low-yield weapons to respond to the Russians using them against you is insane. They are not a form of escalation control, there is no controlling or limiting nuclear war.