system
Given the following post, respond as if you're commenting on Reddit:
human
[Doctor Who] Ashildr is made immortal by technology from an alien race living during the Viking times. Why has no other race recreated this?
gpt
Well the Mire are incredibly advanced, even by futuristic standards of other aliens, and incomparably so when next to the viking era peasants. 

Likely there are two reasons for why they aren't more prominent. Firstly it's possible that the Mire hit the "Coca Cola" of repair kits, in that there's a decent alternative around (albeit also rare), but most other efforts are trash, barely capable of being classed as the same thing. The Mire repair kits are significantly more successful than other healing means, but of course remember that the Doctor had to tinker with them in order for them to work on humans, which would suggest the Mire kits are specifically for Mire alone (unless you have a super smart alien around to customise it for you). 

Then there's the possibility that other races *do* have similar kits. But due to any number of reasons (from BigSpacePharma wanting conventional meds to sell better, to resource shortages, to laws against them) they are incredibly rare and hard to find. Then other races aren't exactly the most chemically and biologically minded. You wouldn't have the Daleks figuring this stuff out in a lab, just like Sontarans would rather stumble upon an enemy that's making these and then claim them as spoils of war (rather than producing them themselves). 

It's also extremely likely that the Mire are the *first* race to make something like this. Without sharing the knowledge of that one thing that makes the kit work, other races are scratching their heads for that missing piece of the puzzle and how to find it.