Why not take moral upper hand and first destroy all of your own nuclear weapons?

Lack of nuclear weapon.

You can’t prevent them from developing a nuclear weapon if you wait until they have it.

They were enriching uranium near weapons-grade levels. What more evidence do you need without seeing an actual assembled nuclear weapon?

I mean do you think the Iranian government is more incentivised to build a nuclear weapon before or after??

They've been pursuing nuclear weapons for decades, and bit by bit getting closer. A nuclear armed Iran would result in the rest of the middle east - most of which considers Iran a serious enemy - to pursue their own nuclear weapons program.

The goal was to stop their progress, not reduce/increase incentives.