Why was NASA's Artemis mission postponed?

NASA postponed the Artemis launch. The reason for that is, that they wanted to be sure that this mission will be successful. It would be embarrassing if the Orion mega-booster will detonate on the launch pad. The key element in the Artemis mission is "Orion". That booster is the most powerful rocket ever created by NASA. And this rocket involves brand new technology that has never been used before. 

So Artemis is much more than some moon program.  It offers a framework for many R&D projects.  NASA can use the "Orion" booster in heavy missions. And one of those missions is to launch heavy satellites in higher orbiters. Also if NASA makes a manned Mars flight in the future, that mission requires a heavy booster to rise nuclear reactors to the Moon orbiter. 

If the nuclear rocket will put together and launched from the Moon orbiter that denies radioactive debris fall to Earth. But that kind of mission requires a heavy-duty booster. 

The U.S. military is concerned about the possibility that low-orbit satellites will be good targets for ASAT weapons. And therefore, the military authorities want to raise reconnaissance and GPS satellites to higher altitudes. 

But if the satellites are used for photo-recon missions that require larger optics. And those satellites are extremely heavy. Orion boosters can use to launch those large-size satellites to higher altitudes, where they are better protected against ground-based lasers and ASAT missiles. 

Artemis is a new concept. It has powerful AI and solar panels. Which are decreasing the use of fuel. The advanced power supply systems are making those capsules more capable than ever before. 

When brand new high-pressure technology is tested. Engineers must make them using very high accuracy. If something goes wrong there is the possibility that the booster will detonate. And that thing is not very good for missions that are planned for Orion and Artemis. 

Images and sources: https://www.space.com/news/live/nasa-artemis-1-moon-mission-updates