Because while not a Bethesda game, it's still something they put out on their engine and something they should've monitored feedback from. Obviously if we're putting out a seqeul to an, at the time, wildly popular game we should take notes on how it's changes were received and apply some of that to fallout 4 or skyrim (of course they didn't) there's a lot of lessons to be learned about new vegas even if bethesda are incapable of good writing making your highest difficulty something that's actually fun to play or making skill checks transparent and include all skills not just speech.
Or how about fixing 3's baffling decision to make "end game weapons" a skill that's impractical to choose as a specilization by just integrating it into the other weapons. I often see people claim that new vegas merged small guns and big guns but the reality is that small guns became guns and big guns got distributed to guns, energy and explosives.
But again, none of this matters fallout 4 has a very poorly designed world, overfocuses on playing minecraft with your town adding more tedium to the return trip and completely stripped skills from the game to remove any of that pesky planning. I guess my main point is that their games have gotten worse in many ways even when shown better ways to do things and its not just the writing.