I don't even know that this is true, though. People used to read out letters to each other or let other people read letters all the time. I don't think there was ever really an expectation of absolutely privacy, especially among relatives and spouses.
Not absolute privacy, but a much higher barrier to exposure, due to the fact that text and email are more easily duplicated than letters. If you want to share a letter, you have to read it out loud to someone, or physically give the letter to them, which takes effort and access. An email can be duplicated with a single mouse click, and sent to as many people as you want, or posted on Facebook. And it's way more easy to accidentally see something on someone's iPad or a shared computer screen than to accidentally open and read someone's mail. Plus, I don't think I've ever met someone who hands over their mail for the kids to play with, while every parent I know passes their kids their iPad or phone to play with.