Gallajorn is a random drop out of a pool of all exotics/legendaries. Whilst Thorn is in a pool with just the bounty exotics. You can maximise your chance of getting an exotic bounty by doing all vendor bounties(that includes Eris and Petra) on all 3 characters. Then you get a selection of 3 exotic bounties. I've only got 2 Gjallahorns since launch from drops versus a lot of exotic bounties, enough where I got them all 300 and 331 individually and did the Thorn bounty for fun but I mainly delete them.