Because of the themes included in the game. No reviewer will want to have his name spearing on twitter giving a low score for a game with such themes and being chased by cancel culture mob. The same thing happens with movies.
TLOU DLC and UC4 DLC both would fall in what you want to put on "such themes" and still...
Left Behind metacritic 88, so even the average was lower than 90 (7 points lower than TLOU)
Lost Legacy metacritic 84, even lower (9 points lower than UC4)
The reason TLOU and UC4 got great scores were becaue they were great games, and the DLC while great and even having more content than several full AAA games while being cheapter "standalone DLCs" aren't on the same scope and have much more focus on themes that progressivism like still that didn't prevent the score to take a big deduction.
They were DLC's and usually DLC's always get a lower score anyway so it would never generate the controversy that a full game would generate. Also I see no controversy with the lost legacy, it's just a girl power DLC just like FFX-2.
TLOU and UC4 were definitely great games and deserve every praise they had, although looking in retrospect I have a few things in UC4 that today would make me bring the score a little bit lower. TLOU may be top 5 or even top 3 for me all time and I wish I could erase my memory to play it again.
Anyway, considering the themes of the game, the leakage, everything is happening in the world right now (hell even Gone With the Wind got cancelled) I don't think any critic would give a score lower than 9 even if they wanted to. We shall see.