Fallout 3 is barely played these days, it's the least popular 3D Fallout (on Steam it's 24 hours peak is barely 1600 players vs 18 000 for NV, 22 000 for 76 and over 50 000 for 4; on consoles it's probably even more skewed in 4's direction), so I think that's why it's doing better than 4.
Almost everyone who was critical of it when it came out has already forgotten about it, while most people who loved it don't tarnish their nostalgic memories by replaying.
Similarly with Oblivion vs Skyrim (600 vs 30 000 players).
Don't forget that many (if not most) people playing Fallout 3 on PC these days do it using TTW. Many consider TTW the only way to play FO3 now, so the number of FO3 players also need to take into account some of the FNV player numbers. We also have to consider that many people play FO3 from GOG and Epic Store (especially since Epic gave both FO3 and FNV for free several times already and the last time was quite recently, IIRC).Whatever novelty Fallout 3 had is long gone at this point, so what's left is a pretty bad game with horrid gameplay. New Vegas being played by many more people makes sense since it's the much better game that has far better content.
Fallout 4 might just have even worse writing than Fallout 3, but the not complete shit gameplay mixed with the settlement building pleases the casual players much more than whatever Fallout 3 does.
I have the numbers and I can say that the TTW installer was downloaded more than 140.000 times in the last 3 months. So even if only 1/10 of those downloads are people actually playing TTW it's still more than 14.000 players of FO3 (in the last 3 months) that will never be accounted on Steam (because TTW runs on FNV and not FO3).