Why not? Probably because of the following Edict (point 3 in particular):
Imperial Edict #666, issued by Her Glorious Majesty Empress Janeway
'We, Empress Janeway, Ruler of this Vessel, Defeater of Borg, Savior of the Ocampa, Whose Glorious Auburn Power Bun's Mere Sight Instills Fear Into The Kazon, etc., etc., etc.,
To all who shall see or hear this read, salute! Do know: As We have considered that
it is better for Our future Career that Starfleet Headquarters not find out certain things
It is thus that We, having heard (and ignored) the Council of our Trusted Lapdog Chakotay , and in consultation with Our Own Enlightened Self-Interests, have approved and understand
1) All mentions of Tuvix shall henceforth be erased from all logs. All logs that don't sound natural with these omissions shall be deleted completely and irrecoverably. This also includes this very Imerial Edict after initial promulgation, to erase all traces.
2) His name is taboo in any conversation whether professional or private or any future log; as is any indirect veiled reference to him
3) No holoprograms of him shall be made (We are watching you closely, Doc and Helm Boy!)
4) Infractions will be under pain of a total memory purge, which the EMH will perform. Unless it's the EMH himself of course, in which case I'll do it personally. I've done so before and ... oh wait, that's still several years into the future. Never mind that.
Given by Her Imperial Majesty, Kathryn Margaret Janeway, by the grace of God and other entities, of her vessel, her shuttles, her crew, and some hapless aliens we encountered along the way, Empress, on the 7th Decimal of the Stardate of Our Great Bird of our Galaxy 49695.
(It's what you get when you're a captain that's been out of touch with Starfleet Admiralty for too long, being answerable to no one.)