Generally speaking(if you're applying public opinion):
FO3 going MAXIMUM GREYSCALE AESTETHIC with a story that literally[THIS IS NOT A JOKE]
chastises your MC for "not sacrificing themselves like their parents did"
just rubbed people the wrong way. Then there's the DLC that had to give a post-story journey to absolve the
"YOU WERE SUPPOSED TO SACRIFICE YOURSELF" moment.
Additionally, many felt the game was too "shooty" for a FO game.
By comparison, NV brought back the color and zanyness many like. More energetic character personalities and groups to engage with.The story having some small semblance of openness also helped(picking either of House, NCR, Legion, independent
or [DLC] Sierra Madre Invasion
). NV felt like Fun Fallout again instead of soggy, sloggy greyscale Zzzzzzzzzz.
If you're going by public opinion of course.
https://gamefaqs.gamespot.com/a/user_image/0/0/1/AAdFc_AAApBB.jpg
https://i.imgur.com/Em9Xlbu.jpg [#Life&Hometown] https://i.imgtc.com/790csbr.jpg