Technically, LSB mandates RPM as the standard package format, so deb packages would be non-standard.

For a more serious response, if you look at the architecture diagram, the fwupd daemon is independent of any packaging or download mechanism so there should be nothing preventing you from calling it from dpkg postinstall scripts.

Probably because firmware vendors are not accustomed to working that way. Maybe they are more willing to submit their firmware this way? Time will tell.