Officially, it was switched because SLS is unavailable. The program had been trying to get away from it due to its enormous cost, but was only able to get this

Officially, it was switched because SLS is unavailable. The program had been trying to get away from it due to its enormous cost, but was only able to get this change through Congress when the inability of SLS to fly more than once a year became obvious. The vibration issues were also very real.