Everything is about their target audience. Middle aged women love Pinterest, senior business professionals opt mostly for Twitter, and young boys and girls spend their days swiping pictures on Instagram (or Snapchat).
Think about the product they sell, and the group of people they try to sell it to, and this should help you to pick the right channels.
Alternatively you can say that in your opinion all social channels are important. You would experiment with each and every channel, at least with a small budget, trying to find the winners, media and campaigns that convert. In reality, a combination of simultaneous campaigns, ran on different platforms, typically brings the best results.