It's really not as uncommon as you think. For example, Microsoft advises that Microsoft Office 2013/2016 can't be installed on Windows XP either. Many pieces of software with an emphasis on the user's quality of experience will just straight-up prevent it from being installed on unsupported platforms.
A game like WoW that goes through very regular software updates (with a huge emphasis on persistent, ongoing support rather than support at one point in time) doing this isn't surprising. Especially when you're actively selling a subscription to the service - it might create some odd legal situations about their obligations to support a product that they're distributing (new patches).