The thrive of Urshifu-S in usage alongside Rillaboom and also Magnezone gave Slowbro a tough place in the metagame. Urshifu-S is just better checked by Toxapex (as a bulky water, since it fears no type-disadvantage) and either Clefable or Mandibuzz as a partner. Rillaboom and Magnezone give Slowbro also a trouble on the field, since Magnezone can just use a strong Choice Specs-boosted Thunderbolt on Slowbro or it can Volt Switch out to Rillaboom. That combination is very common in the metagame and Slowbro has a lot of problems with it. Furthermore Dragapult is very common and can just dish out a status on Slowbro and can use a strong Hex in the process. Dark-types like Zarude are also in a great spot at the moment and Zarude doesn't fear much except a burn from Scald, which it can heal with Jungle Healing. Aegislash also sees a small comeback, which is 1 more Mon, which Slowbro fears, especially the Choice Specs-Set, as a Stab Shadow Ball will just dent through Slowbro. Volcarona is also really common, which is another Mon Slowbro fears to face. So in overall the thrive of usage of particular Pokemon like these hinder Slowbro to do its job and make it that it has a tough time on the field. Other bulky waters are just prefered currently as they either have no type-disadvantage in the case of Toxapex or have a decent typing to begin with in the case of Primarina, which also is a great Setup-Sweeper behind a Substitute.