Well, because it makes no sense to try to increase Q plasma; to what end? As pointed by others, you can increase Q plasma at the expense of Q total, thus precis

Well, because it makes no sense to try to increase Q plasma; to what end? As pointed by others, you can increase Q plasma at the expense of Q total, thus precisely optimizing for the wrong objective (fusion can be produced and studied without looking at Q plasma or Q total; although reporting Q plasma along with the time that the reaction was sustained is helpful). I give you an analogy: let's make the most power-efficient floating point unit; but let's focus on Q_float, instead of Q_total; at the end we would end with a simple very wide adder; if we want to multiply two numbers, the system will convert them to their logarithms, add them, and then use exponentiation. Yes, the floating point unit consumed very little power Q_float was great. Well, I guess you get the point.