Why do you have to pay every month for World of Warcraft?

It costs money each month for playing World of Warcraft because Blizzard Entertainment is a business. They want to make money off of their game.

It also costs money to keep the servers up, pay for tech support personnel, game masters, lawyers, even just electricity. All those cost money.

It World of Warcraft were to go to F2P (Free to Play), then they would have to make money by selling more than just vanity items in their store.