Hackers look to make money—and credit cards are a great way to do this. Hackers look at targets like Magento because it's so widely used. If they find a vulnerability, they can likely deploy this across many websites to increase their gains.
It is probable that there are discovered vulnerabilities waiting for the "perfect" time to be released.
Remember that these exploits are not the type of the past, where the home page is defaced with obnoxious pictures. These are subtle with the hacker (or their software) making repeated visits to the server, obtaining more information on each attempt. You will likely find out months or years down the road—when it's too late. Having to contact your customers thanks to a data breach is not a pleasant task.