This is a good idea, but some users might want to modify or need to modify the command line to boot, if it was signed using fedoras key, then you cant do that. Also some users dont like keeping their trust in fedora and would like to modify their kernel freely. Also, though the private key is something attackers want, if they can read or write the private key, then they can just as easily modify systemd, and get root, or install ssh with their own keys, at that point secure boot will not help you.