Glaxo never completed the phase 3 trials to get dutasteride approved as a hair loss treatment. They must have considered that it was not worth the expense involved in completing trials and marketing it, considering that there was already Propecia/finasteride that does exactly the same thing as dutasteride (although slightly less potently so), and that Propecia itself had disappointing sales. Also there's the fact that dutasteride logically has worse side-effects than finasteride, because it almost completely inhibits action of the type 2 5-alpha reductase enzyme, and also partially inhibits action of the type 1 enzyme which plays little to no role in male pattern baldness and which also happens to be present in our brain tissue.
I guess Glaxo must have decided that the costs of trying to market dutasteride as a hair loss treatment outweighed the benefits in the end.