Because everyone is busy, and the users probably always were busy. The thing to figure out is why they were using pinterest before but now they aren't.

That means what's changed is now it's not enough of a priority to use vs other things in their busy lives. The next interesting "why" question is not "Why are you busy?" but something like "Why isn't pinterest valuable enough for you to use, given that you are busy?"

If you want people to use your product, you need to focus on things you control (making the product more valuable to users) rather than shrug your shoulders and give up at things you can't control (the fact that your users are busy).