It’s expensive. When you’re allied with countries that already have nuclear weapons it’s much better to spend those money on conventional weapons. Nuclear weapons isn’t something that’s to be used. It’s only a deterrent against opponents that have nuclear weapons.
I think that one of the reasons why Russia is losing the war in Ukraine is that they have spent a lot of money on nuclear weapons. They have had no use of these weapons in a conventional war.
Technically you can only learn history since everything we know happened in the past.