According to game's producer Tokuro Fujiwara, their idea was to combine 2D and 3D graphics for Tomba! 1, but they weren't able to achieve they effect they wanted. They went with 3D for the second game so that they can achieve a greater freedom in expression. Also in Tomba! 2, the graphics may have changed from the first one, but it's system and gameplay were carried over so players wouldn't feel alienated from the first game.
Contributed by ProtoSnake