Edit: This turned into a bit of a rant, so the short version would just be that SLS is too expensive even compared to existing rockets, let alone compared to the bleeding edge of rocket technology.

SLS is not reusable, each unit requires several years of lead time to produce (so any mission on it needs to be planned years in advance), the engines were designed for reuse (part of why they're some of the most expensive engines ever built) yet are thrown away after each flight. The upgraded mobile launch tower for the upgraded SLS version has also already cost hundreds of millions without anything being built, and the current launch tower, which would only be used 2-3 times already cost $1B despite being a modified Space Shuttle tower.

Furthermore, the SRBs make the vibration environment so bad they have to make screens in Orion flicker in sync with vibrations for them to be readable. Europa Clipper would've cost an extra $1B to make it robust enough for SLS (as Congress was initially pressuring them to do), it was enough justification to just go with a Falcon Heavy instead.

It's extremely telling that these orders for SLS aren't coming from NASA requiring them for certain missions, but rather Congress legislating that NASA must use SLS. For example, a few months ago Congress was trying to require that from 2030 onwards, NASA fly SLS for cargo at least once a year. However, even ignoring Starship for now, SLS is $2B per launch and is extremely unlikely to have that drop by much. The upgraded version is expected to be able to carry ~130t to Low Earth Orbit. Falcon Heavy can carry 63t to LEO at a cost of $90M per launch. On the extremely pessimistic end of price estimates for Vulcan Centaur, it can do 27t to LEO for $200M. Thus it's much cheaper to do multiple flights of either of the two than to do a single SLS launch.

Then on top of that, Orion itself is too heavy to get to the surface of the Moon and back anyway and a Lunar variant of Starship was the only lander which best met the requirements and which NASA could afford, so the program still relies on Starship being successful. Starship has pretty much none of the issues of SLS, it's much cheaper (it'd honestly be hard for any rocket to be as expensive as SLS is turning out to be), reusable, doesn't have the vibration issues of SLS by virtue of being fully liquid fueled (although this also isn't an issue on Vulcan Centaur, which does use some small SRBs) and is meant to refuel in space, significantly expanding its capabilities.

SLS is in a bad spot compared to even other non-SpaceX rockets owing to the potential of in-space refueling. Congressman Shelby had previously banned mention of 'depot' in relation to space exploration (threatening to outright cancel the entire space technology program if they were brought up) because of how bad they would be for SLS's future. ULA had also done a study and proposal for an orbital fuel depot in 2011, due to which Boeing tried to get the physicist who was leading the project fired because of the risk to SLS. As a result, Vulcan Centaur's upper stage is effectively the same stage as was proposed in 2011 but without refueling. Comically the word depot was even censored out in NASA's final statement on the selection of Starship for the HLS contract.

To refer back to the price comparison, upgraded SLS can do 48t to the Moon. But if you can refuel in LEO, you have the option of launching up to 63t with Falcon Heavy or 45t with New Glenn, then launching a transfer stage and fueling it up with another launch or two, allowing you to get the Lunar payload capability of SLS at close to a tenth of the cost.

Private companies can see that the approach of the gigantic expensive one-time use rocket is not going to scale up, which is why pretty much every company is working on reuse and mission extension capabilities like orbital refueling.