SLS is a stunningly overpriced rocket that Congress is forcing NASA to build to keep former shuttle contractors and NASA centers employed. Congress is sending ~ $1.8 billion every year to states including Alabama, Florida, Utah, and Louisiana to develop it. There are very few payloads manifested for it. Launching the Europa missions on it would be a tremendous waste of money—the benefits would not warrant the cost.
It would be worth exploring the lobbying performed by SLS contractors including Boeing and Orbital ATK, as well as that of people close to the main NASA centers involved with SLS, the Marshall Space Flight Center in Alabama and the Kennedy Space Center in Florida.