TBC had huge non-linear dungeons as well, such as Karazhan. I think you're looking at Vanilla with rose tinted glasses right now. Most Vanilla dungeons were linear corridors as well. Some were short, some were filled to the brim with trash (especially the level 60 dungeons where you didn't need the XP anymore) and a few of them were real dungeon crawls of any notable worth. TBC didn't alter that formula much at all tbh, just made the leveling dungeons less trashy in general. OTOH TBC dungeons weren't banal tank-n-spank shit like everything in Vanilla was since they made the encounters more mechanical and interesting.
It's not rose-tinted glasses. Karazhan was a raid, wasn't it? Was it even a regular dungeon? In vanilla, you had a whole bunch of non-linear dungeons, besides Blackrock Depths you also had Stratholme, Blackrock Spire, Maraudon, Wailing Caverns, Uldaman, Dire Maul, etc, which were to at least some degree non-linear and complex. In TBC, most common dungeons were a corridor.
Quick overview of the instances in TBC and how linear they were:
Hellfire Ramparts: Corridor with a fork at the end leading to two bosses.
Blood Furnace: Corridor with a slide back to the start.
Shattered Halls: Corridor, no backtracking option.
Slave Pens: Appears to be vast and open, is actually a wide corridor.
Underbog: Corridor with water.
Steamvault: Corridor with a fork.
Mana-Tombs: Corridor with backtracking.
Auchenai Crypts: Corridor wiht no backtracking.
Sethekk Halls: Corridor with backtracking.
Shadow Labyrinth: Corridor with no backtracking.
Arcatraz: Corridor with no backtracking.
Botanica: Corridor with no backtracking.
Mechanar: Corridor with no backtracking.
Old Hillsbrad: Actually very open and large, yet requires a very specific and linear series of events to complete the dungeon.
Black Morass: Open but pointless as it is a Defend the Dude mission.
Magister's Terrace: Corridor with no backtracking.
So that's two (being generous in one case) dungeons that are not corridors. Raids on the other hand:
Black Temple: Very large and open, multiple skippable bosses.
The Eye: Short and corridor-like, but most bosses are skippable.
Gruul's Lair: More than a lair for the boss than a full-on raid. Still is quite linear.
Hyjal Summit: Linear, but very large and open.
Karazhan: Utterly massive and non-linear, with many skippable bosses.
Magtheridon's Lair: Again more of a lair than a full-on dungeon. Has only 1 boss.
Serpentshrine Cavern: Corridor, but with a pseudo-linear corridor in two directions branching off of the main corridor.
Sunwell Plateau: Corridor.
So when a raid does it better they do it a LOT better, but even then quite a few are still very corridor-like.