3D graphics require more work and (very arguably) artistic skill. My experience with doing 2D (specifically pixel) art is that it has a much lower floor than other art styles. Using small and stylized sprites doesn't require you to understand anatomy and composition nearly the same way as making a fully rendered 3D model. For artists who are just starting out and want to make artwork for their game but aren't particularly good at drawing, pixel art is something you can dive into and make relatively quickly. Of course, making really high-quality pixel art still requires a substantial amount of talent and skill.
Plus, the turnaround time is a lot quicker. I am doing all of the artwork for my game, and using pixel art means that I can create and revise artwork much more quickly than if I opted for 3D.
But I grew up on those old NES and SNES games, so admittedly I'm just a sucker for the style.