If they cant or are unwilling to, you get to draw two cards, propelling you with either extra cards in hand or putting you at a better advantage on the board.
Perhaps players thought it would be a shoe-in for the Eldrazi precon, or else the main set, so were holding off in case the price dipped
Meanwhile, in most situations, the opponent will be casting whatever they didnt discard and building a lead on board which youll then need to respond to, taking the pressure back off their hand
Senus return to combat when a legend attacks isnt even that exciting
But Battles can be attacked
His character has morphed and evolved and shifted and changed over the years, and the one constant is typically that he tinkers on stuff