Why? Eye strain. Specifically, stereoscopic 3D fatigues the eyes more than normal, says Bashcraft of Kotaku:
The anecdotal conclusion I’ve reached is that [eye strain] is the result of having two screens. The top screen can display the 3D effect, while the touch screen is in 2D… This focusing, re-focusing and then focusing again on each screen could be the cause of [eye strain] I’m having.
The other issue is that the 3DS requires a sweetspot for the 3D effect to be viewed. So you must stay in the same position to view the 3D effect… This makes it difficult to relax ’ like listening to music and constantly have to turn up and turn down the volume. You can’t focus on simply playing.
When the 3DS launched, many Twitter users complained of eye strain, so others do seem to be affected by the 3D effect. I’m finding particular difficulty with the Professor Layton game ’ reading text in 3D hasn’t been fun… causing my eyes to work hard to produce the effect.
Despite turning off 3D, which is optional, Bashcraft says he is happy with hardware. “The 3DS doesn’t need 3D, and neither do I.”
Wouldn’t it be ironic if the majority of 3DS owners ended up turning off the effect?