Slowking is usually overlooked in UU in favor of Slowbro, whose higher physical defense lets it defend against dangerous offensive behemoths such as Darmanitan and Victini. However, there are a few specially offensive threats in UU, most notably Kingdra, that make Slowking worth looking into.
UU Gen 5 Analysis for Slowking ^
It also faces direct competition from its physically defensive cousin Slowbro, who is often preferred for its ability to counter many of the game's powerful Fighting-types and a generally better typing for tanking physical attack
OU Gen 5 Analysis for Slowking ^
So to sum it up? It seems that the typing Slowking and Slowbro possess is better for tanking physical moves rather than special moves. Also alot of the Specially Offensive Pokemon that decimate Slowking do the same to Slowbro - Roserade, Zapdos, Raikou and Shaymin.