A huge part of the reason senior government figures around the world often choose to use WhatsApp or Zoom is to avoid or reduce exposure to the statutory recording requirements using the "official" comms channel would entail. The UK in particular has had serious chilling effects on government due to the fear that email or official communications will later embarrass you following publication under Freedom Of Information laws. This isn't a hypothetical - FoI journalism is a real thing in the UK and a source of a huge number of stories that can embarrass the Government of the day.
As long as the official channels are subject to strong public disclosure laws that allow the media relatively easy access to many government communications, politicians will seek devices that allow them to communicate "privately" even if that means adopting less secure devices and services. The "threat" they see is FoI/embarrassment in press a lot of the time, not the State level Cyber attacker.
I'm not defending this practice, but I can understand why it happens. I've even seen NHS employees (state employed drs in the UK health service) avoid saying things in meetings for fear of the later FoI reveal if the meeting minutes are published.
New blood politicians may know more about tech, phones and services from a _user_ perspective, but most of them are still politicians, not engineers - and just like the vast majority of the general public, will mostly not appreciate the implications of any details bellow the surface of modern technology. What we think of as common sense are based on principles in this domain that is invisible to most people.
There is no simple solution to this. For government specifically, perhaps a new specialised division that dictates policy for technology use throughout the government (I say new because I would not be surprised if some antiquated and dysfunctional version of this already exists).
I also don't think it's reasonable to say it's lack of awareness, the same people wouldn't give the launch codes to a 3rd party, it doesn't require any technical clue to apply this.
UK government and any departments discussing sensitive matters (or everyone, really) should not be using a) off the shelf phones, and b) should not be using public communications networks full stop, nevermind foreign communication platforms.