When muscle tissue grows new cells are created, which require energy to sustain. People generally don't gain fat cells, on the other hand, except in utero, in early adolescence, or for women when they are pregnant. Fat cells, unlike muscle cells, are like balloons that fill up with fat. They do not require additional energy because there are no more cells (I'm not a cellular biologist, but I believe cells contain mitochondria - the power plant - which takes energy from the food we eat so the cell can survive). This is commonly why it is said that muscle is a metabolically active tissue while fat is not.
That being said, it doesn't make sense to me that adding muscle mass doesn't increase metabolism. It's only logical that it would increase base metabolic rate: the more mitochondria, the more calories burned.
Maybe the MDs or cellular biologists out there can help out with this one...