Why did the US invade Iraq when everyone know that Saddam wasn't capable of developing nuclear weapons?

Saddam had been capable of producing a nuclear weapon, and of producing chemical weapons.  He had had active WMD programs in the past.  But sanctions and inspections had hobbled these by the 21st century. 

The War in Iraq wasn't about oil, at least not in the crudest way that critics sometimes allege.  They didn't invade Iraq to steal its oil.  If they had wanted Iraqi oil they could have more cheaply and easily bought it by normalizing relations with Saddam.  Instead it was part of a long standing neo conservative program to revitalize American foreign policy.  Many neo-conservatives, which included people like Dick Cheney, believed that the US had erred by not ousting Saddam from power in the Gulf War of the early 1990s.  They thought that kicking Saddam out of power would result in a host of benefits.  Free flowing oil would be one of them, but they mainly saw this as politically beneficial.  A post-Saddam Iraq, which they assumed would be a free market democracy, would serve as a beacon of liberty to other countries in the Middle East, perhaps sparking a new democracy movement there.  It would also help the security of America's ally Israel by removing a bellicose tyrant from the region.  Finally, they believed that a successful invasion of Iraq would reinvigorate a strong conservative foreign policy in the US, which had largely languished since the victory over the Soviet Union in the Cold War had removed a strong incentive to be active abroad. 

I believe he was going to pull out of the petro-dollar and start selling oil in other currencies. that did not please the US bankers - and they are the ones really calling the shots.

The bankers also profit from wars. They lend money to both sides and have been doing that since the days of Napoleon.

Saddam's own people reported that he had resumed work on nuclear weapons ... and they lied to everyone about how far along the project was.

There were about 40 countries involved with the invasion of Iraq. When has the USA ever won a war without allies?

The talking point about nukes came later, it was chemical or biological they were concerned about. Chemical were found but becouse they were old, or beocuse they originally got them from the US supposedly. They didn't count. Nor were the soldiers who were injured during the disposal of chemical weapons. Nor were the chemical weapons ISIS found in the sand, then used. 

It wasn't just nuclear weapons, it was also biological and chemical weapons which Saddam had already used against Iran and the Kurds.

Plus Saddam kept jerking the International Atomic Energy Agency inspectors around like he had something to hide (which he might of had but perhaps managed to ship off to Syria before the invasion or maybe he was just too clever by half).

The Iraq War Resolution (2002) cited the reasons for invading Iraq.  Oil wasn't listed.

Iraq was an easier target than Saudi Arabia  Pakistan 

Baby boy Bush wanted to steal artifacts from the 🇮🇶 national museum, ancient Babylon is in Iraq, and W. is a wicken. 

Yep oil. ...............

To put down the American left 

To show you can. To spend the money we needed.