So I listened to a podcast about this... but SpaceX and Boeing/NASA are explicitly taking two different approaches to proving their rocket will work. Obviously we know SpaceX is proving their rocket through system level tests. But for SLS the plan has been to use a long accepted NASA approved method of proving their design where individual parts are so thoroughly tested and proven to work that they don’t need many flight tests of the full system. In fact I believe the original plan was one flight test without humans and then have the second flight be a manned mission! So they really aren’t supposed to fail like this.

So at the core are two different approaches to proving their system, where SpaceX totally expects some failures and SLS is supposed to be proven before it even flies. Personally I’m a fan of the SpaceX method but I can understand why the old guard space developers would choose this slower more cautious method.