The presence of suspension on a mountain bike makes the stiffness of its frame a much less important aspect of its ride quality than it is on a road bike. Putting suspensions on MTBs allow their designers to divorce frame stiffness from ride quality, so the frames can be very stiff but giving the rider freedom to tune some aspects of the suspension, such as shock damping and tire pressure. Lock out the suspension, and all that stiffness is there for efficient climbing.