You're paying $60 for the initial launch package,and then $15 a month after for additional content. It costs money to run servers and develop new raids, items, story quests, and more.
People are fine with $60 games that last 7 hours, but a $60 game with a subscription that gives you thousands of hours of content is just downright evil.
Plenty of people are fine with fees, but for some it's a deal breaker. So you are paying 15$ a month, that means I'll only be able to play that month until I get another one. So in that month, it's great if I play it, but that's 15$ lost if I don't. 180$ is a ton of money to a lot of people, especially when you consider that is something that isn't needed and you're also paying for lots of other things that you do need. I could get 3 brand new games for that cost, and in the right combination I could probably even get similar value to this added with the fact that I don't need to rebuy any of those games.
Personally I'd be fine with a monthly fee, but it has to have the right price, regardless of value.