Nintendo has run into problems with converting old 8-bit titles to its '3D Classics' range on 3DS that it didn't anticipate.
Problems apparently arise when you take a game designed in a flat 2D era and convert it directly into a stereoscopic 3D environment.
Nintendo's Takao Nakano used NES shooter Xevious as an example. "With a 2D screen, players had to use their imagination for Solvalou flying above the ground, but on the Nintendo 3DS system, we thought we might be able to recreate it using stereoscopic graphics so it looked like it was really floating," he said.
He goes on: "In the original version, the game unfolds on a flat surface. The moment we made Solvalou float in midair, all sorts of discrepancies arose."
"For example, when an enemy on the ground fired at Solvalou in the original, everything was on the same plane, so it didn't seem unusual if the bomb appeared at the same altitude as Solvalou the moment it was fired and then hit Solvalou right away.
"But with the Nintendo 3DS system, Solvalou is floating in midair. If the bomb suddenly appears - zhing! - at the same altitude as Solvalou... We were like, 'Huh? Something doesn't feel right!' Everything was off!"
To counter this, Nakano says Nintendo has had to make changes to the game and even reprogram the graphics from scratch, which has greatly increased the workload.
Nintendo boss Iwata chimed in: "At first it looked like you would merely port it, but it actually turned out to be a lot of work! I would guess it was about 20 times the work of merely porting it."
Nakano goes on to explain why Nintendo had to scrap plans to convert Tennis on NES to 3D Classics due to perspective issues.
That explains why there's only a couple of 3D Classics available.