Whenever I've encountered managers refusing to use company-provided communications for company work, it's one of two explanations:
(Note these apply specifically to managers communicating with their teams. I'm not including ICs who just prefer to not use company comms)
1) Leadership has developed a reputation for routinely reading people's private communications. Specifically, not just for investigating specific events like harassment claims, but just to surveil people as lazy way of seeing how things are going instead of doing the hard work of actually talking to employees to see what's going on.
or
2) The person is behaving in a way that would be very incriminating, so they force the team to use communication channels that won't leave a record behind. I can think of one manager who was exceptionally kind and caring in company e-mails and Slack messages, but would constantly berate people from his personal Skype account. If you were on his team, you had to be available in both Slack and Skype, and you could usually guess the tone of the communications based on which app pinged you.
I have worked for one, at a time, also. The feedback loop given the lack of pushback from underlings must really foster a condition where such a person thinks that's the only way to be successful, and could even be rewarded for it when they deliver on projects.
I'm glad I don't work in such an abusive environment with my current employment/manager but things change...
(Note these apply specifically to managers communicating with their teams. I'm not including ICs who just prefer to not use company comms)
1) Leadership has developed a reputation for routinely reading people's private communications. Specifically, not just for investigating specific events like harassment claims, but just to surveil people as lazy way of seeing how things are going instead of doing the hard work of actually talking to employees to see what's going on.
or
2) The person is behaving in a way that would be very incriminating, so they force the team to use communication channels that won't leave a record behind. I can think of one manager who was exceptionally kind and caring in company e-mails and Slack messages, but would constantly berate people from his personal Skype account. If you were on his team, you had to be available in both Slack and Skype, and you could usually guess the tone of the communications based on which app pinged you.