Speaking with PC Power Play, Bethesda's VP of PR and marketing Pete Hines talked about Fallout 4's lightning and "bright" visuals compared to Fallout 3's more bleak and grey image. He said:

"The team was very self-aware that Fallout 3 was a depressing-looking world, but that was part of the motif that they were going for, which is, you emerge into this world having lived in the vault and it was very much, your father is on this quest to save humanity. Like, humanity needs saving. Look at all this. It’s horrible. Somebody has got to do something. This has got a slightly different tone to it. And not specific to the story, but just, the mood of the world is more of a, this is life, like, this is normal. This is reality now. There is no more nicely manicured lawns or white picket fences or going back to that. This is what home looks like now."

http://www.gamezone.com/news/bethesda-s-pete-hines-spills-the-details-on-why-fallout-4-is-so-bright-3424207

Maybe because they also figured out that people like their landscape being made up of more than various dark and dull shades of a single color.

Yeah I'd welcome a world that looks slightly recovered while still being mostly apocalyptic. Really felt like the world had ended two weeks ago in fallout 3, it was cool at first and the atmosphere was top notch but the dull browns and greys start to really hurt your eyes after a while.

I'm sure that's why Skyrim was 15 shades of dark aswell. Nice to see colours this time, COD has added colours aswell. Woo!

I much prefer the brightness of Fallout 4 to the bland landscapes of Fallout 3 or New Vegas.

Yeah, they were appropriate, but ultimately uninteresting to look at. I'm so glad this has changed.

I think Fallout 3 was pretty ugly at times. The way they did the urban landscape of D.C. did absolutely nothing for me, and turned me off of exploring.

Also, I hope this titles cityscape is more realistic, Fallout 3 had a very unrealistic rendering of D.C...streets leading weird places, buildings weird in weird spots, etc.

It was only appropriate for New Vegas, because that game literally takes place in a desert. Fallout 3, not so much. Glad they're moving on.

Being gritty just to appeal to the dude Bros is wrong, but in fallouts case I would rather have the bleak colour scheme. It seems more authentic after a nuclear Holocaust. All about context. I sort of like the new look on fallout, but it does border what I would deem too happy vibes

I much prefer the brighter, more diverse palate to Fallout 3's perpetual puke green tint.

Translation - we have decent hardware on the consoles now