Fpr me, the primary reason is that visually it's vast, but in practice, most of these locations are exactly the same. Every ruined building interior uses the exact same set of assets, usually without so much as a single factor to make it unique. The game is basically composed of four different places to explore - a cave, a ruined building, the subway and the access tunnels.
I'm currently replaying Morrowind, so I have a decent comparison - the Dwemer ruins. They look awesome, have a great atmosphere and everything, but except from a handful of locations, most if which are in the Red Mountain area (Dagoth Ur Facility, Citadels Odrosal and Vemynal), they look exactly the same and have the exact same set of generic loot. What's missing is the backstory, something Emil Pagliuro seemingly hates, preferring to tell the story through gameplay, even if it doesn't make any sense in the context of the game world (Vault 106, 108, lights on 200 years after the war, Tenpenny..).
Personally, I felt that Fallout was bigger, it simply felt like a big world. Fallout 3, sadly, doesn't have that feeling, since it's possible to get to DC from Andale just by jogging for a few minutes.