Early episodes that described him as being "an integrated sickbay system" could be viewed as a justifiable reason for this, yet in later episodes we see him being readily downloaded to a mobile emitter, transmitted between ships quadrants apart and then later confirmation that a backup module existed which would certainly confirm that it is possible to make a working duplicate of the Doctor that could run concurrently as long as you had the additional hardware available to do so. Given the impotence of the Doctor to the ship, or the additional benefits of having multiple copies of him running in times of greater need, at the very least it seems reasonable that when they obtained the mobile emitter which functioned independently of the ship they could have simply made a copy of him to run on that while keeping the original running in the sickbay. So why is it that the Doctor could only ever "move" his program between locations, from an in-universe standpoint? Furthermore, what would the point of the backup be if this is the case? A simple workaround could have been to download the Doctor to the the emitter or somewhere else and then restore the last backup to the sickbay systems to restore a duplicate copy? I can appreciate the reasons for these restrictions from a storytelling (Doyist) perspective however I'm not sure how to justify it from an in-universe Watsonian point of view?