Just a quick and dirty way to make money, I'd argue that blizzard would have earned more money by never introducing the cash shop, and making all those pets and mounts available in game, that would keep the game feeling more immersive and players could only get them by playing the game.
Cash shops are disgusting IMO, and the people that pay for the items parade around "look at me I wasted my real money on this digital thing that I put no effort into getting, yay"