This is a ban of those apps on work issued devices, not personal devices. I suspect there's a range of reasons for doing this but I don't think the apps being compromised is one of those reasons. Most likely the opposite, the apps work too well.
Any company allowing the unregulated use of these on their devices has a information protection and most likely also an IT security problem.
Even more valid for any government security or law enforcement organisation.
The reality is WhatsApp and Signal will continue to be high-value targets for exploits given the number of users, cloud infrastructure reliance and promise of secure communications, so it's a wise idea to avoid them for defence matters.