I don't think they ever did switch to full 3D, did they? Certainly, SimCity 4 was still in the top-down style. There was a competing game, Cities XL, which was fully 3D but never really gained much traction (partly because it was pants).
Maxis announced that SimCity 3000 was going to be full 3D, and in fact demonstrated early versions of it to the press, but the project was too ambitious, took far too long, and the technology was not quite there yet. Maxis almost ran out of money before they could ship it.
At that point EA bought Maxis, fired all the people who had been fucking around for years trying to implement the full "VR Cyberspace" experience instead of just the next version of SimCity, reverted to the original design of a sprite based version of SimCity instead of full 3D, invited the reporters who they'd shown the 3D SimCity and explained that now it was going to be 2D like SimCity 2000 but with higher quality graphics, and they finally delivered SimCity 3000.
Going 3D at that time in history meant that the quality of the graphic would take a huge hit, as well as the rendering speed, and fewer people would be able to run it because it would require a high end computer, so it was just not worth it.
Using 2D pre-rendered sprites means that the artists can use as many polygons, rich textures and lighting techniques as they want in 3D Studio Max, and tweak them until the sprites look perfect, and that's exactly what the user sees. You just could not approach anywhere near that quality with 3D graphics at the time. Of course things are a lot different now!
That was during the time that The Sims was also in development. One reason The Sims was successful is that it did not try to be full 3D, and ran well on low-end computers (the old computer that little sister inherits from big brother when he upgrades to a gaming machine). It used a hybrid 2D/3D system of z-buffered sprites, with an orthographic projection constrained to four rotations, three zooms, and only the characters were rendered with polygons into the pre-rendered z-buffered scene, using DirectX's software renderer.
I developed the character animation system and content creation tools for The Sims, and when the EA executives were reviewing the technology to decide if they should buy Maxis, to justify our approach I bought them a copy of Scott McCloud's book Understanding Comics, which explained a concept called "masking" -- http://www.themedianinja.com/glenn/legacy/default_links/anim... ...
Hergé's Tintin comics are a great example of how that works: The idea is that by making the background environment very realistic (i.e. rich pre-rendered sprites from high poly models), and the characters themselves more abstract (i.e. efficient real time 3d texture mapped low poly models), the readers (players) can more easily project themselves into the scene and identify with the characters. Much in the same way an abstract happy face can represent everyone, while a photograph of a person's face only represents that person.
The other fortunate consequence was that it was easy for players to create their own characters and objects by editing the textures and sprites with 2D tools like Photoshop, without requiring difficult 3D modeling tools like 3D Studio Max, so that enabled a lot of user created content by kids instead of professional artists, which was essential to the success of the game.
After looking at the "SimCity Announce Trailer Insider's Look" -- http://www.youtube.com/watch?v=T70evBJE93s -- I have high hopes for the new version of SimCity, and I think it could be better than even the original ambitions of SimCity 3000. The best indication that it will be great is that Ocean Quigley is still the creative director, who's the original art director from Maxis, and he's also the one who came up with the hybrid "holodeck" idea that The Sims used.
He's giving a talk about it at GDC called "Inside the Glass Box": http://schedule.gdconf.com/session/13473480/Inside_the_Glass...
http://www.hardwareclips.com/video/7183/Sim-City-5-Inside-th... http://www.hardwareclips.com/video/7186/Sim-City-5-Inside-th... http://www.hardwareclips.com/video/7187/Sim-City-Inside-the-... http://www.hardwareclips.com/video/7188/Sim-City-Inside-the-...