Every now and then, government officials may be using private comms in order to hide unsavory activities.
However, you really don't need to go looking for conspiracies. In the vast majority of cases, they do it simply because it's slightly more convenient, and they - much like the general public - are either unaware or uncaring of the resulting privacy issues.
The thing is, we, as a society, really do not have a culture of privacy. I do make an effort to protect my privacy, and I think so do most people who read TheRegister's comment pages. But the vast majority out there? They say they want privacy, but only if they don't have to lift a finger for it.
I mean, they barely tolerate having to click an "I Agree/Disagree" button on a website, an action that literally only takes the lift of a finger. Switching messaging app? Nope. Logging in to your secure email? Not even on the table. IMHO, it will take decades before people really understand what privacy means.
Never attribute to malice that which can be attributed to laziness, stupidity or ignorance.