What private info are you sharing with a bug report?
Edit: That didn't come out as I intended. That is an honest question because as far as I know there is no metadata transferred from your app to Signal servers like other apps do. Also, for Signal to stay open source and copy left they have to keep development efforts unencrypted and in the open. Since patches, bug reports, and development efforts are already unencrypted it's beneficial to have email as a bug report channel. If your Signal instance or app is having problems you may not be able to report a bug over Signal itself depending on the severity of the bug. https://github.com/signalapp