The problem is that it's not a little loss of money at the beginning. It's a considerable loss of money, always. If they could make money doing it, why wouldn't they? The reality is that this actually would be a hugely significant project from an engineering standpoint. This isn't a private server. An official Blizzard server rolled back to 1.12 is a much different thing than that. Blizzard may have resources, but they are at a disadvantage here in that they can't just roll out some emulated copy of 1.12 and pay like 3 jamokes to keep the thing on the internet and from catching on fire. Especially if they want people to pay 14.99 for it.
The other half of it is that for all of that work, how many people want the thing and for how long? Probably not that many and not that long. You people keep throwing out numbers like 150, 200k...which actually sounds kind of high to me, but the only important voice here is Blizzard. No one knows better than Blizzard what kind of subscribers this thing would need to be practical to get off the ground considering the resources they would have to dump into it. Then you have to ask, if we do all this work, how long are people even going to use it? How long are people going to play 12 year old content? The answer is without a doubt: not long enough.