The technique known as Magecart is gaining popularity among hackers because of its combination of relative simplicity and efficiency. They don't have to do much more than insert wicked scripts (targeted at remote management and management servers) and wait for people to shop. From there, they can use the information to make fraudulent purchases, make cloning cards, and sell data on the black market.
Don't expect such attacks to go away anytime soon. They have been used against many major brands, including British Airways, Newegg and Ticketmaster. While online stores are not tight against techniques like Magecart, they will be tempting targets.