Because now _this_ party is in power and controls the systems and information, but in 4/8/12 years _that_ party will be in power and a good-willing-mistake-making-bureaucrat may leak these 'by accident/mistake/etc' if they are properly recorded on a gov-controlled system.

But the auto-delete-after-1-week messages from Signal would never be recovered (unless someone is logging all that data and in the future will be able to crack it).