The answer is more political than technical. (Will be some opinion here, alas)
SLS exists as a booster without much of a mission. Well it has Orion, since SLS is the only booster (except maybe Falcon Heavy once man-rated) that can launch Orion.
In Constellation there were several projects.
- Ares-1 - SRB based manned booster for original Orion design.
- Ares-5 - Heavy lift for Moon/Mars/Asteroid missions. (Hab, upper stage, whatever)
- Orion - Actual crew vehicle (Service Module was key as well, since it did a lot)
- Altair - Lunar Lander - aha! A mission!
With the move away from Constellation to the current model, the first and last in my list were cancelled. So now the only mission for SLS is to launch Orion. Orion's mission has drifted and drifted and drifted.
Moon? Mars? Asteroid? Who knows these days.
The way SLS and Orion were structured, politically was to keep people employed. (It sucks to say, and is not the 100% complete story, but it has sufficient elements of truth to need discussing).
Thus there was something of an underlying goal of long term jobs. As opposed to a strongly stated mission with a tight timeline. I.e. No urgency, no goal, no real mission.
Thus you have the oddity of an SLS launch schedule, launching vaguely every 3-4 years. That means you have to pay the staff at the launch pad for 4 years, to launch once. That will never be cost effective.
Worse, they will never be as 'good' at it, as a launch team that launches on a regular schedule. So now you likely need more time and people spent on training to make sure they stay ready.
But by developing it in this fashion, they ran out of money for funding other projects that would lead to missions, so they never quite happened.
It spirals out of control.
This is why government large scale projects have weaknesses. Compare with Commercial Cargo and Commercial Crew projects.
There, NASA wrote a spec, let companies bid, set milestones they had to meet, and paid upon their meeting milestones. Finally paying per mission after mission success.
It is beyond question that Commercial Cargo was gigantic success. Helped SpaceX get over the funding hump, to bring out a new commercial booster that is likely to bring launcher dominance back to the US for the first time since the 1980s. But they get the cargo they need to the ISS, cheaper than they could do it themselves.
Antares was an interesting choice, but since all launchers seem to have a failure every so often, (Sure Atlas V is doing well so far, and I only wish it well, but reality has this way of biting you in the tooshie. Same is true of Falcon 9). But it was actually considered a conservative approach. Existing developed engine (if old, but Aerojet Rocketdyne claimed they could refurbish them back to new) backed by an established rocket company, established first stage developer (Yuzhnoye Design Bureau, makers of Zenit booster), upper stage from ATK (Castors have long flight history), Service module from an establish satellite manufacturer, and a pressure vessel from Thales Alenia (made almost all the pressure modules on the ISS US side). But remember, it is still rocket science.
Will CCtCAP (Commercial Crew) work out as well? One can hope. You have a now established vendor (SpaceX with a rocket with 10+ launches, and a vehicle that is an upgrade from a multiply flown version) and Boeing (the definition of 'established' in the space industry) with a new capsule design but on a booster with a very rich heritage (Atlas V with 50+ consecutive successes).
Will SLS work? Hard to imagine how, except through largess. But if SLS goes away, Orion goes away since nothing else can launch it. All those jobs go away, so will it happen? Hard to say.