The reason is... it's not cardio. Bummer, I know. I love Yoga!! This is one of those workouts you have to consider a "bonus". Remember that just because you aren't burning tons of calories doesn't mean it is not exercise. It's just not something that is going to force you to increase your calorie intake! Definitely track the minutes you do on your QuickTracker and you will get credit for the minutes.
And just do what Yoga does for you... Make you feel good about you!!