The reason Blaziken is banned outright is because Speed Boost Blaziken is released, therefore he is legal in standard play. Since Speed Boost has far more competitive viability than Blaze ever will, good players will prefer that ability for it, but the consequence is that it becomes almost impossible to stop with its terrific mixed attacking prowess, not to mention a huge power boost to Hi Jump Kick this generation that led to Blaziken as a whole being deemed too powerful.