I wonder what the numbers are of subs that will stay with DirecTV because they added channel XYZ in HD versus adding "new" sports HD.
From the DirecTV perspective, the number of subs in isolation is not the correct metric. What is important is the ratio of cost per sub to number of subs. If the cost is low, then even if the number of subs is low, it might make sense to add the channel. If the cost is high, then the number of subs must be very large to make the channel worthwhile to DirecTV.
I am sure there are people within DirecTV who have made these assessments. If DirecTV felt that they could get enough new subs (or retain enough subs that will actually leave) to offset the cost of the PAC12 network, the PAC12 network would have been added. DirecTV knows how much they are willing to pay for each channel. If they can get the channel at or below that price, they will add it. If they cannot get the channel at or below a price that they feel will improve their profit margin, they will not add it.