I don't know about heartstone. But for itb, there's not a lot of information to show at the same time and few system that interact with each other. I think the game design was constrained by the goal of having the simplest possible UI. So, even if ITB is not a 'casual game', it's not very complex and each game is short.

In its core mechanics, it's not that much simpler than, say, a recent Fire Emblem. You can google your way to interviews with the authors where they talk about the amount of time and fiddling it took them to get the UI to where it is. The design evolution of FTL's UI over the years is another good case study. This doesn't just happen by making the game 'simple'.