At least to me, it's the $15 US monthly fee for subscription that causes the failure. It's just too much, that pricing model is at least 14 years old.
I'm betting $10 US price mark would increase the subscription base of most if not all mmogs by 30% to 50%. I don't see a single company trying it, out right. I've seen some like LotRO try it as a discount for 3 months, which does seem to be successful.