BRG is a creature-dense sacrifice/value deck packed with small bodies like Gene Pollinator, Greedy Freebooter, and Biotech Specialist, making Vicious Rivalry's sweep effect very strong against their early board: a modest X catches all of those cheap creatures along with their artifact tokens (Treasures, Clues, Landers). Strategic Betrayal is weaker against their wide board because they choose which creature to exile and will give up the least valuable one, so it is best early or when they have only one or two creatures. Its real upside is exiling their graveyard, which cuts off Lively Dirge's reanimation and their recursive value. Mosswood Dreadknight is a 3/2 that is easily answered by their cheap removal (Requiting Hex, Stir Up Trouble) and would also get caught in our own Vicious Rivalry, so it comes out, and Thror's Map is too slow against this wide board, making room for the sweeper effect.