Well the installer should've given a reason why not but ...... gdebi has issues in that it doesn't call "gksu" anymore in the gui interface....so if it didn't ask for a sudo password, then that is the problem.
- If I heard correctly there are some safety issues with "gksu" of late.
So try installing from the CommandLine using a sudo prefix i.e "sudo gdebi Mullvani" , where you'll have to either give the full path to where the Mulvani.deb file is or "cd" into that directory before issueing that command.
